@lkinny : I know that I am super lucky and the exception with good flex. I never considered it as “only”, but as wow! Granted, my measuring is improvised and not perfect, but here it goes: once I have gotten the knee nice and loose when laying on my back doing heel slides, I bring the good knee up into the same position (heel distance from butt). I slowly sit up, double check that I did not cheat, then relax the TKR leg and use a compass to measure angle on the good knee (anklebone to hipbone, with a thin long dowel where the pencil would go in the compass, so it is fairly accurate in positioning). Then the angle can be measured with a protractor. Another indicator would be that when I kneel on the TKR leg (pillow, of course!), I can get my skinny butt to within 3” of my heel with foot flat on floor. On the other hand, it took me 4 months to get full extension, and I still work every day on keeping it! Go figure!
Regarding water exercises for knee bend: I was given a program that involved sumo squats (wide stance deep squats), front bent knee raises toward chest (fast up, down slow), and knee bends (facing pool wall lifting heel to butt, again, fast up, slow down), and lunge walks. I think those exercises did help a lot in making the knee more flexible.