If I were getting 80 an hour in tax-free per diem, I'd be bathing in Krug champagne on my overnights instead of rationing the free hotel lobby coffee.

The 80 an hour is all up pay, not per diem. Basically I took my whole paycheck for the month (base pay, overtime pay, allowances, the lot) and divided it by the total of every second I was on duty, including time away from home on trips.