Dave,
I recommend an investment in 'Mental Math For Pilots' by Ronald McElroy.
As one advert describes it: "Study such subjects as fuel planning, temperature conversions, reciprocal headings, turn radius, crosswind components, time-speed-distance problems, calculating true airspeed, the 60-to-1 rule, plus many others needed to sharpen your mental math skills."
A quick google provides many possible sources for it.
StbdD