i usually measure the scene and pick an aperture,the flash by itself would dictate. The shutter speed is irrelevant, aperture priority in a way.
Ralph is correct. However please be aware of the maximum synch speed especially with focal plane shutters or else your shutter might cut off part of your exposure in the shot. Aesthetically, I try to keep the flash about 1 stop less than the ambient light. Your taste might be different. Do some test first. On a Vivitar 285 you can set the auto mode on a fixed f/stop then let your camera set your shutter speed. I find Vivitar 283 and 285 guide numbers are pretty accurate. You can also set it on manual mode and read the f/stop calculator.
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