Another idea is to have two modifiers: one for lower speed and another for fast speed:
For example, we can have a modifier for speeds above or equal to 500 knots that could be 15% and another modifier for speeds above or equal to 1000 knots that could be 30%.
Slow speed: 400-500-600 knots or whatever with a Ph of 5-10-15% or whatever.
Fast speed: 800-1000-1200 knots or whatever with a Ph of 10-20-30% or whatever.
Logically, at the tool slow speed cannot be higher than fast speed.
Another idea is that target speed depends on missile speed: if target speed is equal or higher than half missile speed (+50%) then Ph would be 10-20-30% or whatever, if target speed is equal or higher than a quarter missile speed (+25%) then Ph would be 5-10-15% or whatever.
This tool will allow us to play with speed (cruise, reheat and afterburner) and with altitude (low, medium, high).
Personally, I prefer the second idea.