I have to say, for us, the dining plan has always been a great deal. We've paid for it a couple times, and we've gotten it free 3 times. I compared the different discounts we were likely to be able to get, and staying at a Value, obviously the free dining is the best discount out there. At a moderate, it beat out a 40% pin code just barely, and at a Deluxe, you'd be better off with a good room only deal. But honestly, I'd rather do the plan because I just like knowing that all my food is prepaid, and I don't even have to think about the cost.
I'd be a little worried about estimating your food cost, not using the
DDP, factoring in sharing meals. Yes, there is a ton of food and you certainly COULD share, but what happens if you never want to order the same thing? You may find that you don't share meals as much as you thought.
Like someone else said, I think you might want to bump up the numbers on the adult bev/tip column. Maybe $50 on tips, and then figure $9 per adult bev (they are usually around $8, plus a tip to the bartender). Why do they charge so much for drinks?

I'm always defending Disney's prices to people, but I do think their drinks are high. Although, I don't drink much at home, so maybe I'm just behind the times. Do we pay $9 for a margarita everywhere now??