The last time we went in 2009, there was an excel spreadsheet that someone shared with me (and others here on the DIS). You could put in about what each person's meal would cost, after looking at some of the sample menus, and it would calculate your totals. Then at the bottom it compared to the cost of the dining plan and the deluxe dining plan so you could decide what was better to do. Is this spreadsheet still floating around? I'd love to get it again. I'm sure I could do something similar on my own, but it was really nice.
. Should be able to get there through my user info though.

It really seems to be a psychological thing for a lot of people (myself included). I really struggled with it when I was comparing b/c I'm similar to you. I always go for the cheaper options, share desserts (if at all), stash some PB&J in the backpack, only drink water, etc. So the DDP would make me relax over the prices. On the other hand, it creates a different kind of stress - I think I'd feel the need to "get my money's worth" out of the plan and would wind up eating too much and gravitating to the more expensive things on the menus. <sigh> It's just the avid couponer/frugal person mentality.
I'm hoping that getting it for virtually free also frees my brain to just eat what I wanna eat without pressure in either direction!! It's tough being neurotic. 