i use a string line and a speed square: just have our helper and a labor hand hold each end of your string a u stand were your fiting is going to be

mitering a 90 the way that i do it (may not be right but works for me) measure the lenght of the back of the 90 and the inside, divid both by 90, then take that measurement and multiply it by the desired angle, then mark your 90 then take your string and hold it tightly from point to point and have your helper mark it

for what its worth i will tie my fiting in first and leave one joint off and tie it in last by doing that u can get a little flex out of the pipe (depending on the size)

hope that helps
zach

you can get 2 squares  @ each end of 90 degree- set speed sq.-squared off where both squares meet & mark your angle. or measure back & throat , lets say you need a 56 deg. on a 12" 90 measure back is 39 multiply by 56 = 2184 divide by 90 = 24.2666 measure back again marking & 24 3/16 , do the same w/ throat -wrap around w/ steele tape whala you got a miter. easier way theres a black book w/ all the degrees & measurments  + take-offs just can't recall which one it is.hope that helps..