Eagle chain B-gap adjustment without plastic tool
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.