The Lost Art of Running Screw Pipe
I'd like to also add, that the example in your question is slightly vauge, given that you haven't mentioned where you want your 10" measurement to end up in reference to the 90. End of 90 to end of pipe? In that instance, i'd measure the 90 at the 10" mark on my tape where I want it to end up, whether it be end of pipe, center of fitting, end of fitting... etc etc...
Then i would note the measurement on the end of the fitting where i want my pipe to screw into, and add 3/8" to it. That should be the length of nipple you need to make up your overall dimension, wherever you're hoping it ends up at.