Since the PID calculation will probably take less than 5mS why not just calculate it during one mains cycle... you've got 8mS to play with at 60Hz... and if you miss one cross-over point, simply wait for the next one 8mS later... even if you miss ten cross-overs (80mS), it's no big deal in Heating... there's probably so much Thermal Lag nobody would notice anyway.