You can deduct 80% of $63 ($50.40) for every *full* day you're out on the road.
The day you get home, and the day you leave home, you can claim each of those as 3/4 days on the road.
If you're out for 18 days, then home for 3:
Day 1 (Depart): 75% of 80% of $63 ($37.80)
Days 2-17: 80% of $63 ($50.40 each)
Day 18 (Return): 75% of 80% of $63 ($37.80)
(Short cut: Subtract 1/4 day off each end, and claim 17.5 days. It's a lot easier.)
Total: $882.00. I don't know about you, but I don't spend that much for food on the road...
As for the 80% and the $63...
The IRS comes up with a number that applies to everyone ($63), based on their own secret formula. Then, they apply that to us based on our occupation. Most business travelers only get to take 50% of that number. Since we are rather limited in where we can stop, the IRS has been a bit more generous with us.
And nobody gets 100% of the $63...