How do you calculate the maximum watts for the main studio flash strobe?

The main issue most will face is not having the ability to turn the flash down far enough if you get a high powered strobe.

But having more power allows you to use larger modifiers, and it allows you to use lower power settings for faster flash durations (freezing motion w/ IGBT strobes) and quicker recycle times.

For most typical type fashion/beauty work I would say 600WS is completely adequate as long as it has enough adjustability (and consistency). It might be actually more than you generally need; but unlikely to be less than you will ever require.

I would first choose the aperture based on the span of depth of field desired. Let’s assume you want this to be shallow. Start with f/5.6. You can adjust as we proceed. Let’s do with a two light set-up. This will be a main placed high and off to one side. Next place a fill at about lens height, placed somewhat near the camera.

With the only the fill operational, shoot a series of test shots using the watt-seconds adjustments. Let’s pretend the 750 setting produced a reasonable exposure.

We want the fill to be subordinate to the main. Try a series setting the main a 1000 then 1200, then 1500. Examine this series. Use the ISO setting to tune the exposure.

Your series will display a contrast change. Choose the contrast result that suites your needs.