My simple rule for flash use is the combination of f8,GN,ISO at 10ft(3m) and I'll work any power or distance adjustment based on that initial setting.
Having said that, for fill use, obviously I have to find a good combination of shutter speed+f8+ISO for ambient without my shutter going above the flash sync speed. Normally, half the maximum sync speed.
If my flash syncs at 1/500 then my ideal combination would be 1/250+f8+ISO at 10ft. Based on this, especially with that combination of 1/250 and f8, then my GN and its power output is dependent on the ISO.
For example, if the ISO is 400 and assuming the ambient light would give me a shutter speed of 1/250 or near it, then the GN would be GN13 @ 1/1
or GN36 @ 1/2 ISO 100
or GN13 @ 1/4 ISO 1600
or GN18 @ 1/4 ISO 800
and so on
How much light the fill needs is of course dependent on how deep the shadow is. Once, I get my GN+f8+10ft+ISO+shutter combination, then it's just a matter of fine tuning by moving closer or farther by a bit.
Hope this helps.