If you want to be accurate, you can use a flash meter at a fixed distance from the bulb using each of the reflectors with the same bulb size in both measurements.

Once you know the difference in stops between the two reflectors you’ll have all the information you need for any bulb size.

For example, suppose the smaller reflector gave you f/8 + 0.6 stops and the larger reflector gave f/8 + 0.1 stops.

Then the difference in stops = 0.5 = Δf. The reflector-to-flash meter distance is d.

The difference in guide numbers is

ΔG = Δf*d


Suppose that d = 12 feet. Then

ΔG = 0.5*12 feet = 6 (in feet)