I am a spreadsheet maker, too. This year, I did a few different things:
First, I created a spreadsheet of every breakfast, lunch, and dinner. I looked up menus for each restaurant, determined what we would probably order, added tip, and determined a total. (I do this for every trip to create a budget). I am pretty conservative - figuring we eat sandwiches or yogurt and fruit in the room for some breakfasts or lunches when we are at the resort, water at meals, no dessert, etc.
Then, I totaled what we spent on food OOP on our trip last year. It was WAY over budget from that trip. So I created a spreadsheet for that trip and determined that if we'd had DDP, we would have spent LESS money for possibly MORE food. (Where this got weird is that DD was two, so she couldn't have had DDP. She eats like a small human being, though, so obviously we bought kids meals for her. When I calculated, I calculated in the cost of a DDP for her, since the ultimate goal was to determine if DDP may be a value THIS year, now that she is three.)
Next, I created a spreadsheet of where we would eat and what we would order if we were on DDP. I called it the "Hypothetical Dining Plan Matrix." That was fun because I put in some super expensive entrees, and desserts, and pricey character meals we would do off the DDP. The cost came down to about what we'd spend to add the DDP, so we just went ahead and did it! I also booked those more expensive character meals, which is super exciting. We wouldn't be going to Tusker House AND 1900 Park Fare AND Garden Grill off the DDP, but DD will love them all.
One thing that aided in our decision is that my parents are traveling with us (via a separate reservation) and they (well, my mom, really) like to have the DDP. So once we looked at the costs and they were equal, the ease of having all of us on the DDP helped seal the deal. Also, as I mentioned before, DD is a good eater. If she has a typical Disney Mickey Check kids meal (sandwich, fruit, yogurt), she will eat the vast majority of it. She's not very picky about food and likes a lot of things, which helps, too. (Hopefully this doesn't change before April - you never know with a three-year-old!)
I should add that my mom is one of those people unconcerned about maximizing the value of her credits. She absolutely loves the convenience and the all-inclusive, package feeling you get on the DDP, so she always buys it.