This comes up a lot here. If you do a quick search of the boards you will find lots and lots of discussions about it.
It used to be a deal, but for most it no longer is. You do find people who enjoy the convenience of it, but they are often paying for that convenience.
In general the DDP works out if you have children under 10 and do a lot of buffet/character meals. The only way to know for sure is to run numbers just like you have started doing.
We don't use it. We have a "disney adult" child, age 12, who is perfectly happy eating a kids burger and fries. But even when we had two under 10 it never worked out for us. We don't eat like that. We like to share meals or have an app for a meal.
This has described us in the past... 2 kids under 10, and we like to do buffets and character meals. Although we have only done the dining plan when it was free, we would have definitely at the very least broke even had we paid for it. Now that our oldest will be 10 in August, I don't see any chance we would pay for it. My kids (9 & 7) both eat so little that they often share a kids meal when we go to a restaurant. Unless we have free dining in the future, our table service meals will decrease significantly, and we would be much better off paying out of pocket for a kid's meal for our daughter rather than paying $40+ dollars at a buffet.