When you shoot off axis from your flight vector you have to have either a adjustable sight system, which the FDSL did not have AFAIK, or a set distance where the gun and the sight are calibrated to compensate for the speed of the aircraft so that the more the gun is pointed off vector the more compensation the sight adds towards flight vector. The input values that change are then the gun angle and velocity of the plane. And what would complete the system would be the input for the distance of the target, or alternatively you can just use the set maximum effective range of the gun which would generally be OK as long as the angle from you flight vector stays small. When the angle gets bigger and the target gets closer the error is also amplified.
This kind of system was in computerized form in B29, along with a gun position control, and it can be crudely done also with a mechanical system.
You can compare this to early wind-vane sights which were meant to compensate for airspeed when you were shooting sideways. I do not have details of the FDSL system but I'd imagine that it works with this principle, with or without the distance input, which would still generally be better than what the gunners of the era were capable of without any sighting aids.
-C+
