Brooks b17 edge question

[Kitsune06]

On what surface do I do the tinfoil exercise? Something squishy like a yoga mat on top of something firm?

Don't worry, it's not your fault I don't know my sit bones' width.
!

I used a pretty firm, flat surface. I crumpled the tinfoil so the little bumps and crinkles would flatten out where I was sitting on them and give me a really definite answer. Think maybe a wooden chair or counter top edge? Toilet seat lid?

There's a post by Knot somewhere in the annals of the Brooks thread (I think?) that says you should basically subtract 10mm from either side (or 20mm total) from brooks' official width measurements to get the max sitting space, since the outer 10mm is all rail in the back. The B17 is 170? 177? mm wide, so you have roughly 150-157mm of sitting space on it, if I understand right.