I always use this website...
http://www.distripplanner.com/
You can input your ADR's and pick a place for everyday of your visit and it will estimate your cost of food. It will show you what it would cost for the dinning plan vs OOP.
I usually figure it the way I normally eat. The way with DP (dessert, snack and mug) and see the difference. That helps me determine what I would be paying OOP normally and OOP like I would eat on DP.
Personally I never buy breakfast from Disney unless I doing a character breakfast. I ship from
Amazon food for breakfast items or bring from home. That helps cut cost!