You should keep in mind that flash exposure has nothing to do with shutter speed and everything to do with aperture. In complete darkness and a manually set flash, f/4 and 1/2 sec will look the same as f/4 and 1/125 sec. As long as you are at or below your camera's flash sync speed, you can ignore shutter speed for flash. Or to look at it another way, use aperture to control overall exposure, and shutter to control the ambient portion.
I do a lot of on location strobe work, and I'm mixing strobe and ambient all the time. So here's how I go about it:
First is conceptual...how do I want my subject to appear against the background? If its at the beach near sunset, I'm probably backlighting them with the ambient and then making up the difference with strobe. If it is at a park or perhaps a leafy background, I'm not trying to fight the ambient on the subject, but just change the direction of the light so that it's more pressing.
So if I'm at the beach, I put the sun to my subjects back and then ask myself: do I want a bright, overexposed background? And do I want a natural look on my subjects faces? Then I'll want my ambient light *on my subjects* to be either +0 or -1. I'll take an incident meter reading at subject position, pointing back to the camera. Let's say it is ISO 400, f/8 at 1/125. If I want barely noticeable flash and super bright background, I'll set my camera exposure to that. If I want a little less background and a little more drama on the faces, I'll set exposure to -1 using the shutter speed (f/8 and 1/250). Then I'll measure the flash component, and adjust the flash power until it reads f/8.
If I want total drama, I'll set the ambient on the subjects to be -2 and then set a main light and fill lit. Otherwise IMO the shadows on faces can b a bit severe.
So it's measure ambient first, decide how you want your subjects represented by the ambient, and then adjust flash to create proper exposure. You have to balance things like your camera sync speed: many SLRs can only sync to 1/250 or less, while leaf shutters can sync at any speed (but are limited to 1/500 or so).
I suggest checking out the strobist.com site for learning off camera lighting. Having a way of seeing your results instantly is a great way to learn. I will often bring a dslr to check my lighting positions before shooting portraits with flash. I don't use it to meter...that's what meters are for. I use it to make sure the face shadows look right before shooting.
I use a Minolta flash meter. The "IV" I think. It's cheaper that sekonic, which is the de facto standard..