There's no set formula because different manufacturers set up their PSUs to provide different current to different rails. For example, a top-end overclocker's PSU would require more power directed towards the motherboard and graphics cards, while a server system wouldn't need the graphics card juice, opting for being able to power more hard drives instead.
Easiest way to find out is to google your power supply. Ratings are normally posted on the manufacturer's site, and if not, there's a chance that some retail sites might list the ratings.
I just noticed that you updated your post, I was looking for a new post with the specs, oops.
Rather than spit out the specs and doing some simple math for you, I thought I would link you to a test of 11 PSU's, in which it is included. The basic data for wattage and current is there, with some additional information that is not on the sticker. I think you might find the additional information relevant.