A published “Guide Number” is just an approximation. In most cases they work however you can do some fine-tuning.

It’s easy, your revision will take into account any modifiers such as room size, ceiling height, wall/ceiling reflectivity, diffuser type/size and or reflector size/type.

Setup a typical situation, perhaps has a friend model for you. Let’s say it’s a portrait in your living room, subject distance 10 feet. The published Guide Number is 160. Calculate the aperture setting thus 160 ÷ 10 feet = 16. Use f/16 as the basis for this test.

Now shoot a series bracketing this value. A full stop series would be f/22 – f/16 – f/11 – f8. If you want, you can further refine the test using series based on 1/2 f-stop or even 1/3 f-stop..

Now evaluate each image, say the f/11 shot was best. Calculate your revised guide number which will be based on your situation (room/equipment). That will be the f-number used multiplied by subject distance, in this case, 10 feet X f-11. The math is 10 X 11 = 110. This value, 110 is your revised guide number.