How do Pros determine Flash output setting?

For off-camera portrait photography, how do professional photographers determine the power output level of their flashes?

If I understand correctly, light meters will tell you what aperture to use in order to get a proper exposure for a given light output level. But what if I want to adjust my flash output level to a desired aperture value? Will I have to trial-and-error?

Once you know your lights and modifiers, you get pretty good at guessing. I can usually get it right to within a stop or so.

But it is a trial-and-error process right? Assuming you're using non-TTL radio triggers.

I shoot non tll all the time. You will know what settings to use for your environment once you been have done almost everything possible before

It can be trial and error until you learn that say, 1/2 power at 10' = f/whatever.

The good thing is, it's a constant value (provided the modifiers used are the same.)

A hand-held meter makes life much, much easier though. It's a solid investment.

Jamesino wrote in post #12314979
For off-camera portrait photography, how do professional photographers determine the power output level of their flashes?

If I understand correctly, light meters will tell you what aperture to use in order to get a proper exposure for a given light output level. But what if I want to adjust my flash output level to a desired aperture value? Will I have to trial-and-error?

You've already given yourself the answer: Light Meter
If you want your lights to be f/8, then you do a test fire of the flash and get the reading with the light meter, then you adjust the power of your lights from that point until you get an f/8 reading on your Flash Meter.....

The Loft Studios wrote in post #12315050
You've already given yourself the answer: Light Meter
If you want your lights to be f/8, then you do a test fire of the flash and get the reading with the light meter, then you adjust the power of your lights from that point until you get an f/8 reading on your Flash Meter.....

Exactly.
If you can precisely control your strobes (such as Elinchrom or an Einstein in 1/10 stops), pretty much all you do is meter and adjust your strobe output power up or down as needed to reach the desired aperture.

Trial and error is one approach but if you don't have a flash meter its far more efficient to use Guide Numbers: Guide number divided by flash to subject distance = aperture.

Then as you become familiar with your lighting equipment you learn that the Guide Number listed by the manufacturer is frequently overstated so you make the necessary adjustment. Its also easy to test each light and find its true Guide Number; and write it somewhere on the flash for reference.

And just so you know there's some method to the madness; the Canon 580 is called that because it has a guide number of 58 meters; the 430 has a guide number of 43 meters (that's at ISO 100 and flash zoomed to 105 mm setting). Its slightly different for monolights since there's no zoom setting but you quickly learn how to adjust as you become familiar with your equipment.

Jamesino wrote in post #12314979
For off-camera portrait photography, how do professional photographers determine the power output level of their flashes?

If I understand correctly, light meters will tell you what aperture to use in order to get a proper exposure for a given light output level. But what if I want to adjust my flash output level to a desired aperture value? Will I have to trial-and-error?

3 ways from what I can tell. You can either use math, a meter, or trial and error/experience. I am just starting in this, but am getting better all the time. I set up what I think is going to be correct, do a test shot, and adjust from there. I usually can get it in one or two shots, if not having it dead on first shot. I do want a meter eventually, but for now I am happy with results.

For more on that, see post #3: Tips for Xmas Ball Please

The first metered shot determines all you need to know for exposure! Then it is simple adjustments.

Let us assume you set up lighting and measure f/11 +0.3EV, but you want to use f/8. The desired amount is -2.3EV. So I need to go from current power setting to -2EV (1/4 of current power) and dial in -0.3EV on the variator (fractional EV dial)

If your current power is already low and you cannot further decrease power by -2EV and/or your strobe does not permit fractional EV values, then you need to resort to either ND filter or to increasing distance of light-to-subject.

I know that the Sekonic 338 and above will also calculate ambient vs. Flash ratios as well. Then provide you with the correct reading to expose at that ratio. i.e. Adjust your flash power to whatever and the meter will tell you if the flash is 10% or 100% of the exposure setting. Very hand if you want to reproduce a ratio of light...