It's simple...all per diem on daytrips is taxable. If there is an overnight in the trip it will remain tax-free. I know that it is correct, as all other airlines that I have seen do the same. To answer your question specifically, what I have seen in the past is two per diem tables...one for tax free and one for taxable. Your taxable pay remains separate, I do not know how per diem is taxed...but I think it is about 30%.