Vivitar 283s and 285s are IMHO the gold standard for an inexpensive, rugged auto flash. You should be able to pick up used ones all day, and a new 285 can be had for around $100.00.
An auto flash will give you typically 3 or 4 settings, varying based on depth of field, and will tell you what f-stop to use for a certain setting. The 285 also gives you full, 1/2, 1/4, and 1/16 power settings.
Let's say that your auto flash indicates an f-stop of f8. Now let's say that you meter for ambient light and get a reading of f8 @ 1/125. Set your shutter speed to 1/125. Set your aperture to f11 instead of f8. That's a 1-stop underexposure for flash and a 1-stop underexposure for ambient. This is, in my experience, a good place to start. If, for example, you change your shutter speed to 1/250, you're still underexposing the flash by 1 stop, but now underexposing ambient by 2 stops.
This is the beauty of leaf shutters: your fill flash possibilities are so much greater. I was shooting this weekend with my Canon AE-1 and wanted to use fill flash on a shot. However, with 400 speed film, and limited to 1/60 shutter speed, I couldn't make it work. With a leaf shutter camera, it would have been a breeze.