There are four parameters that are interrelated in the mission:
Delay between each shutter actuation
Altitude Above Ground (ABG)
For example, if you increase frontlap, you have to either reduce drone speed or decrease delay between shutter actuation. If you increase altitude, you can increase speed (all other parameters unchanged) due to the slower movement of the ground in the frame of the photo. Etc, etc etc.
In your reply you state “The app automatically computes the maximum speed the drone can have for a given overlap and flight altitude.” It would be good to know more detail as to what you are referring to, HOW the max speed is computed.
I would guess the formula Pix4D uses to determine “max speed” is based on how quickly the camera can take photos. This can be affected by things such as memory card speed. So of the parameters above, I would guess your formula uses a constant value for delay between shutter actuation. Can you confirm this? And if so, what is the constant value, in seconds, for the Phantom 4 Pro drone (X4S camera)
Your formula, using that constant would then calculate the speed of the ground movement given the altitude and thereby determine the max speed based on a minimum required delay between shutter actuation.
So if the shutter actuation delay cannot be adjusted by the user of your software, what constant value are you using in your max. speed calculation?