I would agree with the consensus that by definition macro is 1:1 but 1:2 is good enough. I think that 1:3 is stretching it though and would really wish that vendors with 1:4 and 1:5 magnification would say "close focus" or something instead.
As far as formulas go (rustling sound of web pages being turned)...
If you are using an extension tube
magnification ratio = extension length / lens focal length, so a 50mm extension on a 50mm lens should get you 1:1. The longer the extension tube the greater the ratio, as you'd expect. I think it's a 2 stop loss of light in that case.
You can also mount a reversed lens on a main lens using a "macro coupling ring" (double threaded male). The main lens is mounted on the camera normally. The resulting
magnification ratio = main lens focal length / reversed lens focal length, so the shorter the reversed lens the greater the magnification. I have used a 28mm on my FA77 (surely an abuse of that lens!) which got 2.75x.
The Vivitar 2x Macro Focusing Tele Converter (AKA V2XMFTC) turns any attached lens into a macro. The longer the lens, the more the magnification, but I am not sure of the exact formula. I think it's the same as the extension tube one above, so a 50mm gets you 1:1 and the 77mm gets approximately 1:1.5.
Not sure where I got this info from, but I did do quite a lot of reading and experimenting. Now I own two 1:1 lenses plus a 1:3 that claims macro and a V2XMFTC plus reversing rings. That might be enough.