Ask HN: Resources for creating a pro forma?

I'm about to start fund-raising for my startup, which means we're going to need some financial projections. (We're pre-revenue.) I know the basics—start with your core assumptions, see how they play out, think of it as a learning exercise, etc.—but I'd like to know more specifics. For instance, how should I go about estimating fundamental numbers like the cost of customer acquisition?

I'd really like to see some real examples of startup pro formas. There must be some out there. Has anyone who's gone through the process written a good guide, or a detailed reflection on their experience?

I'd also like to know if there are any good alternatives to using Excel. Or is that sacrosanct? Are investors likely to require a spreadsheet in .xls format?