Not sure you're asking the right question. 10x zoom means that the telephoto end is ten times the wide end. So a 10x zoom starting at 70mm would end up at 700mm, and isn't practical.

You may mean converting the focal length to a smaller sensor.

Quote:
Is there a formula for converting say a 70-210 lens to what it would mean in zoom such as 10x or 12x?

The way I read it, this would be a 3x zoom, i.e. 210/70=3 (with a 70mm "wide" end)

With a 35mm film camera a 50mm lens is considered 'normal'.
So just divide by 50: 500mm/50 = 10X . With a digital you
would have to include the crop factor:
(500x1.5)/(50x1.5) = 750/75 = 10X.

70/50 - 210/50 = 1.4X - 4.2X zoom.

/Clay