Koji correctly points out that I naïvely overlooked various factors. With a similar analysis to the wiki page, I get:

• Ideal arm length of 37.795 m
• Ideal PRC length of 6.753 m
• Ideal SRC length of 5.399 m

This means that:

• The PRC, measured at 6.759m, is 6mm long. 
• The SRC, measured at 5.474m, is 7.5 cm long

Next step is to see how this may affect our ability to sense, and thereby control, the SRC when the arms are going. 

MIST simulations and plots are in the attached zip.