How can I determine Rt?

The best way to determine Rt is to back-calculate a value based on impact locations measured in the field.  Keep in mind that Rt, as well as Rn, should be in the range of 0 to 1.  When specifying standard deviations on these values, be sure that Rn and Rt values cannot come out larger than 1.  Values larger than 1 essentially means that upon impact the rock is gaining energy instead of losing it.  This can result in unusually large bounces, and seemingly odd results.