I have run my Hobbywing 120a with a LRPX12 21.5 and my FDR was around 5.2- motor ran hotter than an x11 but nice throughout the whole race. I'd never ran 21.5 before until our last club day. I TQ'd and won all three finals.

Depending on motor of course, i'd say 5.3-5.6. Best way to find your FDR is to run the car for a couple of laps at a time and get a mate to test the temp each time. Heat is the key and adjust accordingly.