When I was in planning stages I did a lot of work with Excel to help us think things through. Set up a little spreadsheet that had variables I could change: price per pint, percent of different types of taproom sales (pints, growlers, schwag), number of seats, number of turns, number of pints per patron average, days open, etc..
Then I set up variables for the brewery: bbls brewed, % split between wholesale, retail, taproom, etc., prices, and so on. Then I set up a little section that was average check, based on the number of pints in the taproom (no food here). Did that 'average' feel reasonable to me based on my own lengthy and personal barstool experience? Would I come back if my two pints were $20? Would it be reasonable to expect each patron will have three pints? Or is it more likely some will have five, some one, and the average will be somewhere around two?
Most importantly I did sort of a 'Reality Check'. Stats like: is it reasonable to assume these many patrons, drinking this many pints, in this many hours open, paying this sort of average check. Messed around with all these variables till I got numbers that I thought seemed reasonable enough. It's one thing to say "We expect $xxx,xxx taproom sales." but quite another to see it as "In order to reach that we'll need to serve 1000 pints daily in a 12 person taproom...Ooops." At that point you can lower your expectations and then you can tweak your keg sales/taproom mix to see roughly how many wholesale kegs you need to sell per week/month/etc. to cover the gap. But you can also find that level where your taproom pays the bills and wholesale kegs start be where the profit comes in.
Find a nice conservative estimate where you can still make it, then be pleasantly surprised when you do better, I say. We open in about a month, so we'll see how accurate that turns out to be...
Co-Founder / Head Brewer
Bainbridge Island Brewing
Bainbridge Island, WA