Due to uncertainty of what half-width and fillet radius to use for the rounded square used for the sweep rail, did a sketch of the various parameters:

Given a FerriteCrossSection half-width,
and given a FerriteFillet, (less than or equal to the half-width),
and given a RadiusOfWire , (which is relatively small):

The half-width of the Square to Sweep can be calculated,
as well as it's Fillet.

Edit: redid the sketch, due to inconsistent names.