I have a 7" flash reflector and the Guide Numbers for the 5, 5B, 25, and 25B bulbs with a 4" to 6" reflector. How do I convert the Guide Numbers from a 4" to 6" reflector to a 7" reflector?

Should I take the area for a 4" to 6" reflector of 19.63 square inches and divide it by the area of a 7" reflector 38.48 and multiply by the Guide Number?

7" Guide Number = 5" Guide Number*(19.63/38.48)

Or should I use the 4" to 6" Guide Numbers?

Steve