I checked the effect of the arm length to the reflectance of the f2(=5*f1) sidebands.
Conclusion: If we choose L_arm = 38.4 [m], it looks sufficiently being away from the resonance
We may want to incorporate small change of the recycling cavity lengths so that we can compensate the phase deviation from -180deg.
f1 of 11.065399MHz is assumed. The carrier is assumed to be locked at the resonance.
Attachment 1: (Left) Amplitude reflectance of the arm cavity at f2 a a function of L_arm. (Right) Phase
Horizontal axis: Arm length in meter, Vertical Magnitude and Phase of the reflectance
At L=37.93 [m], f2 sidebands become resonant to the arm cavity. Otherwise, the beam will not be resonant.
Attachment 2: close-up at around 5 f1 frequency.
The phase deviation from the true anti resonance is ~0.7deg. This can be compensated by both PRC and SRC lengths.