Lumber and structural framing is usually 16" on center, and it is also usually some number divisible by 4, be it 16" - 20" -24"
If the light is to say commercial then the corner tabs are usually for ceiling tie wires that attach to the bottom of
structural support above as the light sits inside the ceiling grid.
Using math there's a lot to be determined by you.
There might be holes existing or dipples to be knocked out for screws or toggles.
You can open up these little holes and just slap the light into position and mark holes. Use appropriate securing technic.
One might have to determine were to drill four 3/16" holes for 1/4-20 toggle bolt in four corners of the light if they're not there already there and
use the available holes with screws or toggles bolts and a washer. You don't have to miss the framing just use the existing holes and send in screws with a washer, it just depends.
If it were me, I'd find the center of the room or determine where one wants the light to be and put a round box flush mounted box to gypsum,
if your using blue boxes or even better a raised round mud ring on a 4 square box. On the light figure out where one could drill or cut that
size hole to the out edge of mud rings, thus the mud ring will sit raised inside the light once it's against the ceiling.
NC inspectors are spastic about having complete access to any box behind a light. FYI
You could just leave a whip up there, but digging it out after sheetrock is problematic, I've left wire double and tripped stapled
in a position where the sheetrocker had to open a hole at near the exact spot, I wanted. I don't like to do that because I'm not there
to put other trades to work. Nor do I trust the other trades will work as to what I'm trying to apply.
It's really compounded if the light in the center of two sided sloping ceiling. There could be from one to three truss's across the room,
so up to 4.5" is gone to the center of the light and one has to determine the work accordingly.
