Robertlorimer- I don't think I've heard the term v.a.t. before. I've always divided the plate cost by the desired Food Cost % to get a rough estimate of selling price. In my experience, you won't have the same FC% or gross profit margin % for all menu items.
You should update your costs at least monthly for you regular menu, and cost each catering job. It'll start to drive your purchasing, especially with produce and center of the plate items. You can even forecast costs alittle especially with meats and poultry.
Since you are costing your menu, I would plug that data into your POS system so it can track your Cost of Food Sold based on POS Sales. (Or you can make a spreadsheet using your menu mix and plate costs) Take that information and compare it with your actual Cost of Food Sold you get when you take inventory. This will help you identify and control shrink, and thus save the all important $'s.