There are established per diem rates for most cities in the U.S. and abroad. I believe that the national average last year was $42 dollars per overnight. You can only deduct the difference between your actual per diem and the national average, if there is any. For example, lets say you get paid 30 dollars in per diem for an overnight. This is twelve bucks less than the national average. Add up all of your overnights, multiply that number by 12 bucks, and then multiply that number by .6 and you get your deduction. Don't ask my why you only get 60% of the difference, that is just what the tax pub states. Good luck.