When you add a close-up lens to a camera which is focusing at the shortest distance at which the objective lens can focus, and you don't change the focus adjustment, the focus will move to a distance which is given by following formula: X = X / (DX + 1)
X being the shortest distance at which the objective lens can focus, in m, and D being the Diopter value of the close up filter. This is the minimal working distance at which you will be able to take a picture with the close-up lens.
For example, a lens that can focus at 1.5 m combined with a +3 diopter close up lens will give a closest working distance of 1.5/(3*1.5+1)=0.273 m.
This is what I found online -- I haven't checked it against what might be in the ASC Manual yet, but if I understand this formula correctly, then if your lens focuses to 4' minimum, then a +1/2 Diopter should change the minimal focus to 1' 4, for example. But I'm not sure I'm doing the math correctly because it seems you could never focus the lens to 2' because every focus distance I toss into the formula with a +1/2 diopter keeps getting me a distance that never reaches 2', which can't be correct.