rebuild 9 1/4 rear end

[j m w]

i have a 9 1/4 suregrip rear end that i want to put a 410 gear in. i have worked on almost all parts of cars, rebuilt 50 or 60 motors, replaced every part on all makes,but i have never put a ring and pionon together. i do have more than a clue about to do it, but i cant figure how to measure the pionon depth. richmon says to set it at 2.706 but i cant see how to measure form half way on the ring gear to the end of pionon gear with the suregrip attached. i know i am missing something. if i keep missing with it i am sure i'll figure it out but i could use some advice.