Do you know of a site that has common ratios/statistics or "rules of
thumb" for start-up software companies?  In other words I would like
to review my financial projectsions versus what are "accepted" ratios
of revenue to various expenses for a growth company.

Hi! This is not specifically for start-up companies but it is within your industry.

Software Publishers:
Average Net Income as % of Revenue = 9.9%

http://www.bizstats.com/corpnetincome.htm

Thanks!