Apologies if I misunderstood your comment, but are you implying the price per share can be different for common and preferred stock, and therefore the spread used to calculate AMT for employees can be negligible even post investment?

I assume you're referring to the "AMT" alternative minimum tax, which is calculated based on the spread between exercise price and current valuation according to most recent 409(a) filing. My impression was that this valuation, calculated post-series A/B/C, reflects the most recent per-share price paid by investors. Since investors typically receive preferred stock, the per-share price in the 409(a) filing will reflect the price investors paid for preferred stock.

In most cases, common stock will not have a real "per share" price until a liquidity event. So how is the price-per-share valuation calculated for common stock?

I was under the impression that the spread used to calculate AMT is the difference between the most recent preferred stock price per share, and the exercise price of the options, regardless of whether they bought common or preferred stock.