DM - Prior to my surgery I did LOTS of pushups. I used to do 3 sets of 60 pushups daily. I'm not bragging, just setting the stage to show that my upper body muscles were well-toned prior to surgery. After surgery, I couldn't stand to do even ONE pushup for about a year. At first it just hurt too much, then over time, while it didn't hurt, it didn't feel "right." I just waited it out, starting with 10 reps around 15 months after surgery. Even now, fully 6 years out, I am not back to the level I was prior to surgery. I do one set of 35 reps daily, could probably do a second set, but as I get closer to 70 years old, I don't feel that I need to maintain that much muscle mass any more.
I've always just done what feels OK, then just a little bit more. No science to it, just pushing my limits gently until I get results.