I came across an estate sale today and picked up an old Spiratone ring flash for a few bucks. But I've noted it doesn't have a table for calculating exposure based on distance. It also doesn't have a button to trigger the flash without firing the camera, so I'm not sure how one could meter without wasting an exposure.

Is there some sort of formula for calculating exposure with a ring flash? Or will I need to just meter it at different distances and build my own exposure table?

Thanks for any ideas!