Effective distances with elevation changes can be approximated by a formula. (Which it sounds like you're working to improve upon), but there can never be a hard rule.A rangefinder cannot calculate effective lengths for disc golf throwing, just horizontal distance and height difference from a direct angle measurement (Pythagorean calcs) which are only slightly different.
Different throwing styles will be affected differently by elevation. i.e. Take two players with equal maximum flat-level distance, but one player throws faster and one player throws with more spin. The spin thrower will have a longer effective distance on an uphill shot than the speed thrower.