Valuing unprofitable growth stocks.

I have been looking to gradually buy beaten down GAAP unprofitable growth stocks.

I am not an expert but believe atleast part of their investment in S&M, R&D should account towards growth capex and hence GAAP may be underreporting their actual earnings.

I have been looking for suggesting grid showing EV/S, Gross margin and growth rate in grid just as a rough guide, better even with reasoning.

My very basic calculation is if GM is 70-80% then on maturity of the business NPM should be 20-30%.

so for EV/S ratio of 5 (80% GM, 20% NPM) may indicate normalised PE ratio of only 10.

Off course, this is very simplified and doesn't count discounted rate for future earnings.

But, if one can buy forward EV/S ratio between 2-3 for 20-30% long term growth and 80% margin, might be a good buy for 5-10 years horizon.

All suggestions and guidance welcome.