Doug; a few more details would be helpful (you say pre-revenue, but do you have customers/users? do you have a product released? have you taken any investment yet - friends and family? . At this early stage the valuation number doesn't have to be generated from an overly sophisticated process. You are primarily looking to pin a number on the valuation that you can rationalized now and down the road with the IRS and investors. If you had to sell the company tomorrow, what would someone pay for it? If you don't yet have customers or infrastructure or patents, or servers, etc. then likely not much thus:
1. as Jon mentioned, if you have recently issued stock then use that valuation
2. if you haven't yet issues stock, but you have invested some cash then use that number.