Generally speaking, when you are at twice the focal length of the lens, you will have 1:1 magnification. Thus, for a 210mm - when the bellows are at 420mm, you have 1:1. It is impossible to say how much it will take to focus at two feet since it will depend on the subject and focal length of the lens. After this, every time you increase the bellows by the focal length amount, the magnification increase by one power; so for the 210 - if 210mm of bellows is at infinity, 420mm is at 1:1 and 630mm is 1:2 (two times life size). Alternatively, 315mm of bellows draw is 1.5:1 sized. You are looking at the ground glass when focusing? Or are you wanting to do a P&S type of thing, where you focus without looking through the glass?
I have found it much easier to use a calibrated focus target to determine exposure increases necessary due to increased bellows draw; some people like measuring but it never seems to work for me in the field.