I would suggest asking the flagger first.
I also prefer new-work cans. I do drywalling, too, and I always get the round and rectangular holes just right. I use a hole-saw for round holes. They just fit too well to do it by hand (quicker, too).
I measure from the butting edge to comfortably just before and just past the opening (box, can bracket flange, etc.) in both directions, and transfer those four numbers to the drywall.
You don't need an exact measurement; within 1/8" is more than enough for square-cornered holes, and just cut inside the lines. I'll slightly bevel the hole larger on the stud side for a closer fit on the outside.
You can be even sloppier for round holes, as long as you allow the same amount of clearance on both sides of the can as you measure in each direction. Use easy-to-see-and-remember fractions.
I draw a square or rectangle using those numbers, and draw an X from opposing corners. Regardless of the "precision" of my drawing, the X marks the center of the circle for my hole-saw arbor.
