I have noticed that in order to use the auto level feature of S3D, a cylindrical print bed of 250mm needs us to entered as a 250 x .707 = 176.75 as the X-Axis, Y-Axis in the machine's build volume on the machine tab of the options menu. This allows auto level to function without probing outside of the 250mm diameter bed.
This way of calculating the build volume introduces a bug with manual support in that you cannot add manual supports on any area of the model that is outside of the 176.75x176.75 defined build volume. For example, if I try to print a model of a 200mm tall guitar laying flat on the bed, supports are needed for the head and neck of the model as they are a few millimeters off the bed. S3D will not allow me to manually add support structures to the part of the head/neck that extends past the build volume in the machine panel.
The algorithm used to define the delta print bed build volume needs to be calculated as a cylindrical bed, not a square bed otherwise many models can not be printed using the available bed size if supports are needed outside the currently calculated build volume settings.