IMC is not such lossy. IMC output is supposed to be ~1W.
The critical coupling condition is G_PRC = 1/T_PRM = 17.7. If we really have L_arm = 50ppm, we will be very close to the critical coupling. Maybe we are OK if we have such condition as our testing time would be much longer in PRMI than PRFPMI at the first phase. If the arm loss turned out to be higher, we'll be saved by falling to undercoupling.
When the PRC is close to the critical coupling (like 50ppm case), we roughly have Tprc x 2 and Tarm to be almost equal. So each beam will have 1/3 of the input power i.e. ~300mW. That's probably too much even for the two OMCs (i.e. 4 DCPDs). That's OK. We can reduce the input power by 3~5.
LO power when PRFPMI is locked is calculated by assuming 1 W of input power to IMC. IMC is assumed to let pass 10% of the power ().