the "adds 5% to lightning damage" takes all your physical damage (and physical damage only) and adds 5% of that as lightning to your overall dps. So if you use a weapon with only physical damage, this acts as a flat 5% DPS increase. If your weapon however has any elemental damage instead (even if it is lighning damage) the elemental portion will not be increased, so you end up with a bonus less than 5%.
The 25% more damage on lighning skills improves all skills, which state, that they cause x% weapon damage as lightning. This improves all damage instances of Fists of Thunder, including the additional aoe component of Thunderclap. Also, it improves the tornados spawned by Sweeping Wind/Cyclone. However, it does not improve Sweeping Wind's base damage, which is still quite a significant portion of your overall DPS, also it does not work for Seven-Sided Strike.
Additionaly, to my knowledge, it will not improve the base damage of your weapon, if it has additional lightning damage.
So you cannot count these bonuses as a 25% or 30% flat damage increase.
Blizzard has recently (un)clarified how +X% elemental damage works. It only adds to base weapon damage. Say, you have a weapon with 500-1000 damage (all physical), and 5% lightning damage, then you effectively get an additional 50 weapon damage as holy. What you get is 2*(min weapon damage)*X% elemental damage, i.e. 2*500*0.05 = 50 in the example.
Now, if you instead have a weapon that has +(200-600) elemental damage it doesn't add to this portion of the damage. The damage type (cold, fire, lightning...) doesn't matter. The same applies to other items that give +X% elemental damage, like e.g. Inna's belt.
Adding the 25% + 5% is wrong, they have to be calculated separately. Now won khim lau itself has (+Lightning damage), so the 5% can essentially be neglected. However if you use a main-hand like echoing fury, which has + min/+max damage and damage%, it is very effective at least for the main hand.
Finally, note that the +X% lightning damage is factored into the DPS that is displayed in the character sheet. So once you own the weapon, you don't have to worry about doing the math for that part of the damage.
Moving on, the 25% increased damage for lightning skills:
This boosts the damage of both main and off-hand, but only lightning skills. So FoT/Thunderclap benefits fully from the 25% buff. Sweeping wind damage does NOT benefit from the 25% increased damage, only the Cyclone damage is increased by 25%, since only the cyclones do lightning damage, not sweeping wind itself.
Now I don't know how much of your damage is done by sweeping wind. Lets be modest and say it's 20% of your total damage. This would reduce the effectiveness of WKL by 20%, so you end up with an effective 20% increased damage. So 50k char sheet DPS would be 60k actual DPS.
The +X% elemental damage is added to 2*(min weapon physical damage) on both hands, and is calculated into the char sheet dps
The +XX% damage to lightning skills affects FoT/Thunderclap and the Cyclone component of Sweeping wind, and is not displayed in the char sheet
Yes i know, but apparently its basicly guesswork to know how good the weapon is, since how do u know when a weapon is better .
How much dps can u loose on a weapon for the won to still beat it.
i loose 10k dps when using the won but i don't notice much difference,maybe 10k dps is not big enough to notice anything.
if i knew exactly how much dps it gives compared to a good rare, then might invest in a good version, but now its just guesswork
i don't see that many monks using the weapon, probably because u just can't tell what it provides..
Have you tried using a dps spreadsheet to compare weapons? I know mine supports both elemental affixes, though it currently doesn't support Dual Wielding. There must be some monk spreadsheets out there that can take those affixes into account since the math behind the "Adds x% to elemental damage" is known, as thundersteele above posted.
The only monk i knew that could do that, kinda stopped streaming it seems, so no, i don't think there are monks outthere with spreadsheets that i know off. Im pretty sure any monk would already have posted something in the monk section if he made something like that.
Alot of people claim this to be an uberweapon, best monk offhand, but i havent seen any support data to actually back those claims with numbers which is whats important.
The 5% elemental damage are easy to take into account, but that's not going to make a big difference. The 25% to lightning skill damage is harder to estimate, since it depends on how much damage each skill contributes to the total damage output.
We can try to make a calculation for single target.
FoT/Thunderclap: 120% of DPS
SW, aoe damage: 45% of DPS (but, depends on whether it's cast with main of off-hand, and if it is cast with 30% Faith in the light buff)
SW, cyclone damage: 20% of DPS (+ same as SW, depends on weapon and buffs when cast)
Very naively, this would make SW's aoe damage account for 25% of the total damage. If we assume that it's cast with 30% buff, it accounts for 29% of your damage.
AoE makes things less obvious. FoT/Thunderclap does 35% AoE in a 6 yard radius. Every third hit in addition hits AoE in a frontal cone. Sweeping wind does AoE for 45% damage in a 10 yard radius. Then, you can spawn multiple Cyclones per hit, so that damage also goes up with AoE.
Since all three damage dealing abilities scale well with multiple targets, it's probably safe to assume that the ratio of 1/4 to 1/3 of total damage being from Sweeping Wind AoE still holds. So I would calculate the damage with Won Khim Lau as follows:
char sheet dps * (1 + (0.15-0.25)*(0.66-0.75) )
the first bracket is the variable + lightning skill damage (max is 25%), the second bracket is an estimate of the uncertainty in the calculation. In numbers, with 50k char sheet DPS WKL adds an extra 8k-10k DPS.
Now, how good can WKL be? The best possible roll is
~980 DPS (unlikely, but lets say around 950)
65% crit damage
100% crit damage from socket
Let's compare this to the best possible Legendary, Az'Turrasq's fist:
~1350 DPS (1300 realistically)
100% crit damage
100% crit damage (gem socket)
So we're looking at 30 dex, 35% crit damage difference, and 300-400 DPS. Dex and crit damage affect both weapons. The dps gain from 30 dex is about 1.3%, from the crit damage it's about 10%. Weapon DPS is 36% higher, but only increases half of the damage output, so it's an 18%. Then, there is the 5%-6% elemental damage of WKL. This affects both weapons, but only part of their damage. If the second weapon has high physical damage, this should top out at 4% DPS increase. In total, Az'Turrasq should provide 22%-26% more char sheet DPS than WKL. The effective DPS increase from the 25% lightning skill damage should be between 15% and 20%, so a bit lower.
Finally we would have to compare with the best possible rare. Compared with Az'Turrasq, such a weapon can have up to 350 dex, which benefits the damage of both weapons, while the IAS on the Legendary only works on that weapon. In the end, I don't think it will be much different.
In the end, I think WKL is a great weapon if it gets the right stat rolls. I would decide against it in the end since it limits your ability to change skills. Since most monks don't bother experimenting that's probably a non issue. My favorite choice is a Echoing Fury with 0.25 hits/second and a perfectly rolled slow rare off hand. The way echoing fury works makes a slow weapon benefit more from the buff.