You could break up 66% into several groups of percentages. 50% plus 10% plus 5% plus 1% ≈ 2/3 pot.

1/2 pot + 1/10 pot

plus half of (1/10 pot)

plus 1/100 pot (optional)

Those are all fractions that should be easy to do in your head. Obviously the 1/100 pot is pretty much a waste of time, since you're already at 65%.