You can obtain a radius constraint by editing
a diameter constraint. Double-click the diameter constraint
and select Radius in the Dimension list in the Constraint Definition dialog box.
If you want to create a formula, note that:
The parameter corresponding to the radius
or diameter constraint is referred to as RadiusX.object.
This parameter always contains the radius
value. As a consequence you can only add a formula on the
radius, not on the diameter (the diameter is a way to display
the constraint, not the parameter itself).
For more information about formulae, see
Engineering Rules Capture User's Guide.
You can add a formula to the radius value of a diameter constraint
using Me > Formula.