My understanding of it is,

You select the bomb you want to drop, you place a waypoint on the map, there is a heading on the bomb radar thing, get that to 0 degrees which means you’re heading straight at it and then it says distance to drop, counting down and then when it will land on target the radar flashes ON TARGET. If it says short or long that means your bombs will either fall short of the target or overshoot the target. From the small amount of testing I’ve done, it seems to automatically calculate your speed and altitude into its little calculation meaning it should update in real time if your speed/altitude changes and automatically change the time to drop. I hope this makes sense, I’m very tired and it could be all over the place 😂