PT6A:
First issue that has to be determined is what the radius of the RF leg needs to be. The FAA 8260.52 has wind tables and TIAS assumptions for that.
Then, where an RF leg joins a TF leg it has to be at the tangent of the TF leg.
The arc center point is simply the "hub" of the RF "wheel." The arc center point has to be included in the database in order for the avionics to be able to calculate the RF leg.
An RF leg can also join another RF leg of a different radius or direction but the rule of tangents must still be observed.