The grid must not warp with the photo when moving the points. It has to stay at a right angle, otherwise you can not judge the effect. Is there a switchover option?
Affinity Photo: When using the tool "Cut" I can see a grid. When rotating the foto, the grid is perfect and fixed, only the foto turns.
When using the tool "Perspective", there is a grid, but this one follows the curser, so you can't see, when the perspective is set properly. You need to make lines before.
Thats not so helpfull.
Perhaps, there is a possibility to toggle?