speed with flash will have to be the synch speed or slower - you need the shutter curtains fully open when the flash fires. If you are using the flash on the hot shoe, into a mirror, I would suggest set the shutter spped at the fastest sync speed - typically 1/60 or 1/100, etc. Then vary the distance from the flash to the bounce surface. Or the nature of the bounce surface; a white wall, or even a neutral grey card would work fine. You just don't want to introduce a colour cast.
To duplicate results, tie a string knotted at the right distance, and a note on the string noting what the flash output was. The string gets affixed to the camera, and then you step back from your bounce surface until the string is taught. Drop the string, and take the duplicate exposure. Keep all kinds of notes for the first time you try this, soi you know which exposure set up yielded the best negatives.
I would suggest if it is an auto flash with it s own sensor eye, to set it for the smallest aperture to start, because in that manner you will chew though fewer batteries. If there is not enough light, then step the flash up to the next larger aperture setting.