I always thought the dining plan was too good to be true.
I found the calculators do a great job of estimating what the cost could hypothetically be, but as others mentioned, the best way is to set up a spreadsheet and pick out each meal for each person. The calculators will not take into consideration if you are ordering steaks at Sci-Fi and Mama Melrose or vegetarian dishes at Narcoossee's and California Grill. The calculators also do not factor in whether you really need a dessert with every single meal or if you would actually separately purchase the amount of snack credits allotted.
If your travel party consists of multiple children doing multiple character experiences and you truly love dessert and snacks, I can see it working out with a savings. My travel party includes all adults who eat 90% TS meals and do not need desserts with every meal. I allot $5 per person, per day for snacks (which we never end up purchasing this many) and $19.15 for a mug each since we do use them and always find I save so much more without the plan in my calculation since I am not purchasing unnecessary food and not tipping on higher meal prices that I would probably order just to feel my credit was maximized.