Establishing sales forecast that are reliable require looking at a lot of different factors. Here are some of the questions you need to ask yourself.
Location: Is the tasting room easily accessible to large numbers of people or is it off the beaten path. If its in a place with good access to people, parking and visible to traffic passing by then you can expect sales to be higher. If the tap room is on the back side of a industrial complex on the outskirts of town with no visibility then driving traffic is going to much more difficult.
Marketing: Have you budgeted anything for marketing at this point in your planning? Marketing consist of more than just running ads in local media but should include special promotions and events especially ones at the tap room itself.
Food: Having your brewery and tap room have access to food will make a huge difference in how much beer you sell. If there are restaurants near by that do take out and you have a few tables for people to sit and eat and drink then plan for higher sales.
Entertainment: Give customers something to do when they're there. Put in a couple dart boards or a bocce ball court to basketball hoops. The more things to do that are in addition to drinking a beer the longer they'll stay and the more they'll buy.
Now that you've figured out about how many people you think you can attract per day to the tasting room assign dollar figure to each one. If you think an average visitor is going to drink 2 pints and or fill 1 growler then average thoe two purchases out and you have your average ticket. Take that number and multiply it by the estimated number of people through door each day and you have your daily sales. Keep going up the ladder until you have annual in store sales.