Your assuming that the grid is parallel to the equator. Otherwise you’re saying exactly what I sad. Supplying a vector would allow you to have an “angled grid”.
True.
My supposition (earlier in the thread) was to assume cardinal orientations, as this was trivial to calculate.
If we were to consider rectangles, on offset angles to the equator, then (as Phil pointed out) we’d be looking at calculating great circles and using them as boundaries.
I’m sure that can be done, but if I was doing it, probably not in my life time. 