From my experience, KOPS is not as critical for mountain biking as is saddle height. Cockpit length is more important that KOPS, in my training as a m ountain bike coach and understanding of mountain biking.

As for saddle height, it's simple. Put your feel out of the clips. Put heel on pedal. Leg should be fully extended but knee NOT locked, but not bent either. This is your climbing height. I find it's useful to take something like a carpenter's awl and make a mark on the seatpost. No, you will not hurt your seat post. For many, the ideal XC-flowy trail- up and down kind of riding seat height is about an inch lower than your marked climbing height. This gives you the ability to be more dynamic, move out of the saddle and so on. Last but not least, you'll lower the saddle even more for pure descents.

Here's another thought. Go to a bike fitter and get your hip-knee-ankle alignment checked. This is rarely checked, most of the time you get fit from the side to check the KOPS. Anyway, if your knee is dropping to the inside or outside of the vertical hip plane ( when viewed from fore/aft), it will cause pain. This can be corrected with an insole by a qualified fitter.