The Endless Walk set paired with a stone of jordan is actually better than F&R, because SoJ also gives discipline. F&R setup = 3440 × 2.25 = 7740% vs Endless Walk setup = 3920 × 2 = 7840%. It's slightly better but harder to maintain full uptime of the whole bonus, which is why most preffer F&R. But endless walk does give you some dmg reduction, it's all about personal prefference.
I didn't bother to finish reading bwcause you were wrong from the start....the F&R rings are MULTIPLICATIVE, that's not how their bonus is being calculated....you even have the details to show this in your char planner. The F&R bonus = 50%*50%= 125% or 2.25 multiplier. This also gets multiplied to your other multipliers. For your example, 3440% + 3440% * 125% or 3440% * 2.25