Shango hit it on the nose. Figure out a weekly or monthly "nut". The problem is, without experience, its hard to get it right before you open. I would add 20% to that number, and be prepared for a 3 to 5 year period before you turn a profit. Hopefully, this won't be the case, but you should be prepared for this scenario. If this doesn't scare you, go for it!
Mike