IIRC, the standard (N) developing time for FP4+ in 1:50 Rodinal is 15 minutes @ 20 C. Pulling by a stop should be less 33%, i.e. 10 minutes, to start with. It may help to develop slightly longer, but start at 10 and test. What is your reason for rating at 64? You are effectively pulling the film and reducing contrast. If that is what you want, then no problem. I personally shoot it at 125 and develop as standard, and like the tone curve as is. One can develop it 1:100 stand, which is a compensating developer then, and it will give lower contrast in the highlights if that is what you want. Not that I do, but if I wanted a slower film with finer grain, I would rather use PanF.