I'm just getting into using studio type strobes and off camera flash (ya I know all about strobist.com and other resources one can search for online).

Short of using some kind of meter to measure the intensity of flash(es) you are producing, how does one know what settings to use (in full manual)? I can guess and probably get pretty close - or experiment to find what works is always an option too...I just don't want to make a lot of big mistakes, especially if I start getting in "model volunteers" to help me learn this stuff.

The variables are so many - light intensity, shutter speed, aperture, ISO, distance - it all seems like a bit of a guess.

If a flash meter is absolutely necessary, what's the very least expensive option to start out with?