No idea if there's a standard.
What I did was a full-lock turn on flat ground, with and without the stopper, and measured the difference between the two maximum points reached by the front wheel.
(Check picture for a thousand words description
Difference is 17cm.
I'd like to remove it permanently, but I'm not totally happy about it. Sharp edges from triple clamp hitting frame =
I'd like something to protect the frame but not as thick as the standard stopper. Will keep looking.
