The rafter plate thickness (rpt) refers to the number of framing members that make up the plate. If there is just one, then the thickness would be 1-1/2", if 2 then 3" and so on. This value is then represented by (rpt). If you are framing a conventional roof system where the birds mouth cut sits on top of the double wall plate of the upper exterior wall then the (rpt) would equal (0) zero.