You have a unit measuring for attack range to another unit at elevation, up on a cliff/building, or vise versa down onto a target:

1) Do you measure from above the units as close to base to base as possible? A two dimensional space measurement really, not accounting for the difference in height, which will shorten the range.

2) Do you measure at an angle from base to base? A more accurate line of sight measurement, accounting for the angle and full three dimensional space.

There does not seem to be coverage for this in the reference that I can find?

I would measure from base to base at an angle but thats just my opinion on how it should be done, I dont know what the RRG might say about it.

I imagine it’s still closest point to closest point. So still a straight line, it just won’t be horizontal.