I actually think I just got it after posting my question. Since the sensory level past that point is not fully intact, you have to use the sensory levels to determine the motor level. If sensation was intact (2s) the whole way down then the motor level would actually be S1. Is that correct? I was thrown off because the motor level on the L side is C8, but I think that is the case because T1 is rated 0/5.