Guess I was measuring from the wrong spot on every door I hung for 6 months
I was hanging interior doors at a flip in Boise last spring and kept getting a 1/4 inch gap at the top, so I blamed the cheap pre-hung slabs and just shimmed the hinges. Then a finish carpenter walked by and asked why I was measuring jamb depth from the outside of the casing instead of the back of the jamb. I checked with my tape and he was right, my cuts were off on every single opening, 11 doors in that house alone. Now I want to know, do you measure depth from the back of the jamb or off the framing, because the two guys on my crew swear by different spots?
The real issue is not the tape spot, it is which plane you are working in. A jamb has depth and it has width, and the back of the jamb can be proud or shy of the framing by a fat eighth depending on how the framer set the king stud. On a Boise flip those rough openings were probably framed fast and green, so the stud face itself can bow after drywall. What matters more is where the door face lands relative to the finish wall, because that is what your eye reads from the hall. Pick one reference for the whole house and mark every opening with it, even if the number changes per door.