Yeah you have to measure. a guess of 8-12 degrees doesn't really do you any good.
I just cut and turned my front 10bolt a few months back. 4 inch lift + clocked up transfercase = I needed 10 degrees to point my pinion at the right place... 10 degrees made things interesting where the center pin goes into the perch, so they got welded on, and they are the perches now...
Also, 10 degrees would have very negatively affected steering castor so I cut and turned the knuckles.
It was a bunch of work, but i'm happy now that eveything is set up properly.
...but I would be MORE happy if it magicaly turned into a dana60 now that it's done.