There are a lot of forms out there yu can use but you still have to understand the process. Take the cost of a product and divide it by the number of portions you will make from the item. Do this for each item individually. I've made up the prices and the portion numbers in these examples.
Package of burgers cost 16.00 there are 24 servings in each package.(16.00 divided by 24 = cost)
Buns 2.50; 12 servings(2.50 divided by 12= cost)
There are some estimates, items you may want to factor differantly:
Iceberg lettuce 16-18 servings (1.60 by 16, leave yourself a cushion, use the low number of portions)
Tomatos 6-10 servings each, but you are paying $2.00 lb. but you are getting 6 toms per lb. so divide the 2.00 by 6 each then divide by any average number of slices you'll get, I'd use the number 8.
You'll have some items that you make an educated guess on. Ketchup, pickles, etc. You can estimate an amount for that or find a barometer to gauge everything by. Such as count the buns packages you go through for a few days, make a mark when your #10 can of ketch is gone; make another mark when the pickles are gone, etc. Then you can check your use againest the estimate you made. (if you make 70 burgers with one gallon of pickles & the pickle cost is $6.30 for a gallon divide by 70 servings; but you served 50 burgers to the #10 can of ketchup. Take the cost of the ketchup and divide by 50 burgers )
You'll get a feel for costing your items and verifing. We always seem to have one menu item that I'm verifying my cost/usage on. Prepping the salad greens, counting the plates used for the evening. The quantity of fries I put on a steak plate are differant than a Burger, so I can't assume that the cost is the same on the fries for both dishes.
I like to keep a page per menu item, and have my calculations clearly labled. So if one items goes up in cost I can go in and revamp a number easily.
I like to factor my base operating costs with my food also. for example, I use $400. nat. gas a month so I divide it into a per day cost. And I look at it that way as I have to have the stove and grill on everyday and ready. But if I'm doing a 100 meals a day my cost for gas is less per plate, than if I'm doing 50 meals a day. Also keep in mind an average for the week, as 3 days maybe less than 100; but 4 days maybe 200+. These numbers are usful to me as I can tighten my costs some, and makes me aware that in my slow winter months my daily operating costs are considerably higher and therefore it costs more for me to produce a plate.