Its not even remotely what you said. Its A/B+1 or A/B-1 for an interior loop.

edit: I didn't need to be this aggressive. It's VAGUELY what you said. its (A+B)/B. You have missed the /B part.. which is A/B + 1.

in the example you gave, for radius 2 and 3... it would be 3/2 + 1 or 2.5. Not 5 (off by a factor of 2 because /B)