Pocket Wizard and Einstein 640

You are going to need to be pretty close to f/4 from 1/64th power. Regardless, the "easy" solution is a ND gel. Otherwise, you can program the MC2 to have a starting point lower than full power, giving you access to the minimum flash output. However, there isn't two customizable settings so you either need 2 MC2's (one for full power and one starting at say 1/4 power as the max) or you need the ability to change the MC2 in the field. I might think a ND filter is cheap, fast and easy. But that's me!