The description of the SB is very clear: 700% weapon damage plus 300% of your shield Block Chance.
So, max value is 20 natural block chance (Piro Marella) + 11% enhanced block chance roll = 31% block chance.
The base final top SB damage is 700 + 3 * 31 = 793.
I don't get how you come to the conclusion that it's very clear. In fact I find the block part so vague it's of no use.
Your "shield block chance" can be up to 75% (unless you have another description for what the final buffed figure is called if not "your shield block chance") or it can be the shields own block chance. How do you know it's the latter?.
Edit: Having thought about it for more than 2 seconds it occured to me that the description is actually pretty accurate. According to it then it must be the full buffed shield block chance that counts, if not they should have phrased it "of your shields block chance".
Shield bash damage only increases from your shields innate block chance, not block chance increased by skills or items. I.e. I have a lidless wall that has 20b I get that base block chance unless the shield itself has a %b on it. because %b on a shield raises the shields Vanilla %b(which is all that matter in damage calculation.)
I am Spanish, and my English is only average, so I am not sure what the exact words would fit better... When I started with the mannercookie fire shield bash build (just when Roland set became bis), I tried all the skills, items and passives I could think that improves block chance, and they didn't raise my dps, so I thought the shield is the only item related with the 300% part of the damage...