Although Flexibility Premium generally varies by the radius of the area worked, officially it has nothing to do with recompense for travel but is to cover short notice changes of shift and not having fixed turns. There is a formula to work out the percentage FP for a post based on the number of booked day/night/spare turns on the base roster, which is why RDR men almost invariably receive a lower premium than GPR men even if they work the same area. For example round my way the RDRs are on 12% and the GPRS are on 24% for exactly the same area, because the RDR men work a base roster with fixed turns and only a few spare shifts (when they effectively become GPR). It's all detailed in the NRP.