I am no mathematician, but if one can gauge the angle of trajectory and distance, the apex should be discernable. What’s the formula. Google it, I reckon. Might even be a spreadsheet cheat.
a small rocket assist can change that.
Most of these long range guns have some sort of system that has an assist similar to a rocket firing from the back to fix some of the aerodynamic problems. That’s how they can get some longer distances. That is what Bull invented.
You take the vertical component of the starting velocity and apply gravity (32.2) multiplied by time to reach zero vertical velocity. Then 1/2 a*t squared to get the height.