What I did was I planned out everywhere we wanted to eat, and I literally went through and tried to guess what we would eat at each place. Being the tightwad loon that I am, I made an entire spreadsheet, guessing at every meal. I feel that I know our eating habits pretty well, so I think I probably hit it pretty close. The estimate turned out as follows: the DDP would save us a whopping $15. *BUT* that $15 savings also gave me peace of mind of going into the restaurants and ordering what I *really* wanted and not having to worry about money. I went ahead and set up the DDP. We now have a little over 4 months until our trip, but our dining is paid off and I know I will enjoy myself more having it paid in advance.
Good luck with your decision!