If you don't have a flash meter, and really like doing math, you can always use your flashes'
guide numbers to calculate the f-stop. This way, you can have flashes with differing output capabilities, set at different power levels, placed at different distances. You can place and calculate each flash individually to get the desired f-stop on your camera
Check your flash manual(s) for any tips on manually calculating beyond the basic formula.
If you use any modifiers (umbrella, pan reflector, soft box, etc...) you'll need to test and find out how many stops (or increments of a stop) each one eats up and adjust for that amount in your calculation.