How do you measure your MPG?

[Redneckrich]

I can drive from Ft. Carson Colorado to Ft. Riley, Kansas (475 miles) on a single tank with 1/8's tank left over from a 30 gallon tank and cruising around 55-60 mpg from Ft. Carson via hwy 24 to I-70 and cruising at 70 mpg. So how do I figure out my MPG? I know going from Ft. Carson to Denver airport and back, I only burn just under a quarter of a tank, max speed is 70. So what the equation that I need?
Note my truck is a 2006 F-250 with a 6.0 bulletproofed engine with a 5" lift. I'm not exactly how heavy she is, but I did add a tool box and filled up with tools, the weight and lift kit doesn't seem to effect the fuel economy I'm getting now, compared with the 2-1/2" lift and no tool box and no tools.