The allocation radius does control the number of groups produced in the non-hierarchical classification in PATN.
Experience suggested however that most people wanted to select the number of groups rather than having a stab at an allocation radius that would produce a reasonable number of groups. Most tended to manipulate the radius to get a desired number of groups.
What PATN does is to use a branch and bound approach to manipulate the allocation radius to home in on the user-selected number of groups. In some cases, hitting this number is not possible.
Is there any reason you want to work with the allocation radius directly?