I think you do learn to guestimate pretty well over time, but a more direct way is to look at the histogram. If your camera is like mine, the histogram has a verticle grid where the lines are one stop apart. So you can take a test shot and see pretty well how many stops under or over you are, and adjust accordingly. And in terms of flash power, one stop is cutting the power in half or doubling it, so the full stop points are full, 1/2, 1/4, 1/8, etc.
I think Arena's method of taking a no-flash picture to get the ambient the way you want it (which may be essentially black) is a good one. Then adjust flash power to get the final picture. But if the flash and the camera are in manual mode, then EC and FEC won't be involved, so I'm a bit confused about that reference in Mcoomer's post.
Of course the formal, correct way to do this is with a flash meter, where you actually measure the light at the subject and set things accordingly. But that's another level of expense, and some excellent photographers, including David Hobby, don't use them, mainly because they say you can just take a test shot and get the same information.
In my experience, if you learn how to shoot in manual, with no ETTL, you pretty quickly learn to ballpark it, and then adjust based on what the LCD picture and the histogram look like, but mainly the histogram.
Actually, for portraits, with the flash reasonably near the subject, 1/4 or 1/8 power is a reasonable place to start, just as a rule of thumb.