When you say "the series-A guys are offering the same stock (roughly) as the series-D", what units are you comparing them in? Number of shares? Percentage of the company? Estimated $ value of the shares (and if so, how calculated)?
When evaluating an offer, you just multiply this number to the projected IPO price given the comparable (same industry, company size, etc) IPOs recently.
Actual share counts are quite arbitrary. It's common for startups that are going public to do a 10 to 1 stock split shortly beforehand. Using your algorithm, an offer from that company would 10x more valuable the day after the split!
Much better would be to compare the expected value, by multiplying the ownership percentage by some estimate of the value of the company.