If your first location is doing well, then that's something that comes from business planning. There's no magic formula. You look carefully at the market in general, the market at the second location, what it would cost you in expenses and assets and related development costs, how much your fixed costs would increase, and how much your sales would increase. Make carefully researched educated guesses.
Tim Berry is the chairman of Eugene, Ore.-Palo Alto Software, which produces business-planning software. He founded Bplans.com and wrote The Plan-As-You-Go Business Plan, published by Entrepreneur Press. Berry is also a co-founder of HavePresence.com, a leader in a local angel-investment group and a judge of international business-plan competitions.