Depends on the light (subject brightness range). 64 works well with 9:00 development in normal to contrasty light, you can shoot at 100 with 9:30 development in flatter light.
Originally Posted by gliderbee
Note that those times are for rotary development with prewash. Without the prewash, you probably want to reduce the times by at least a minute. I use prewashing because I develop in (often wet) Expert drums, for which there is the risk of highly unsightly dribbles of uneven development unless you get the whole emulsion swollen before starting development.