Here is an article about the roof top not turning out well:
The higher something is the less overlap it is going to get. If you say to take 75% overlap at ground level it might only get 50% overlap up on the roof since it is closer to the position of the aircraft.
We recommend using our Ground Offset feature to raise the plane for which the overlap is calculated up to roof level:
https://support.dronesmadeeasy.com/hc/en-us/articles/209895406-Ground-Offset