Steve - Get the So Ezzy calculator on this site and download it - Has never failed us once. Our Minimum is 50, and our "sweet spot" is in the 150-225 Guest range.
Need to look at your food cost, labor cost, production cost (Charcoal, electricity, rub, equipment/vehicle cost/maintenance, insurances, business license(s), supplies, utensils, aluminum pans, lids, foil, etc), gasoline and/or mileage, marketing, and State/Fed taxes and Sales Tax.
At the end of the day, with all expenses allocated as above, your "net" net is what you'll have. Everyone's accounting practices differ. Business plan is a good start, but don't be afraid to re-evaluate and course correct as you go!
Rancho To Go BBQ Catering
Spicewine BBQs - West (Left) Coast
Large Spicewine - Ol' Blue
Twitter - @ranchoagogo @ranchotogobbq