SRAM makes this plastic tool for adjusting the B-gap on their Eagle derailleurs. My question is, if I want to measure this distance without this tool (say with a caliper instead), how should I go about it? Between which two points should I make the measurement and what's the value I should aim for?
This measurement is given by both Shimano and SRAM as the shortest distance between the circles formed by the tips of the teeth of the largest cog and those of the guide pulley.
The spec for Eagle is 15mm.
A simple way of actually doing it is to hold up a 15mm wrench, caliper, track nut, etc to it.
Give it a little fine tuning once the chain is on and you're setting up the shifting. You'll be able to tell how small changes (1/4 turn-ish) have some bearing on how smoothly and quietly it all works.