OPINION
I think KOPS is a good starting point for saddle position, but not the end-all, be-all. At least for me, KOPS or KIPS (knee in front of pedal spindle) is workable for a few cm. KBPS (knee behind pedal spindle) is less advantageous as it seems to rob you of power compared to when the pedal is further back as in KIPS/KOPS. I liken it to the comparison of a traditional road bike vs a recumbent. A recumbent is an extreme example of KBPS so the power generated is not great as a road bike. Pretty obvious on climbs & sprints. The leg just doesn't feel as powerful forward of KOPS.

I think a few cm either way if probably fine. Beyond that, the bike doesn't fit.
YMMV