I quite liked it when he wrote "chimp" ....
But as always some really good structured explanations from Steve, thanks.
I get the impression that most people using multiple off camera flashes nowadays, tend to have gained a personal "feeling" for their particular flashes / strobes and use an instinctive approach to initial manual power settings. For example, at home I work in a small space with my Main light (softbox), fill Light (umbrella) and hair light (bare) very close to the subject (eg 1 to 2 meters away for each light). My three flashes are all similar GN, so for F8 at ISO200 I have just leaned from experience to start with my Main light (softbox) at 1/4 ..... the Fill (umbrella, a bit further away) also at 1/4 .... and the hair light, a bit further away again, at 1/16.
These are repeatable and memorable for each time I use a similar setup, and inevitably lead to only minimal, maybe a half stop, adjustments.
So when I move to a bigger space or outdoors, I still use these remembered starting settings, but mentally adjust for any extra distance (I suppose subconsciously using the formulas and patterns that Steve detailed above). But ultimately we have the huge benefit of the histogram to assess test shots, and I do this with each light individually, and adjust to place each light exactly on the histogram where I want it ..... eg main light about 75%, fill about 30%, hair about 60%.