The code looks fine and the result looks reasonable given the parameters. It looks like the vertices are on the order of about 1 units apart, so a 3 unit radius is not very large selection; therefore, the center-offset is large in comparison to other values because the radius is relatively small.
ahhhh,. i just adjusted the radius to a bigger value and also excluded the Y coordinate during computation of vector magnitude. its now working good, thanks for your help Bob!