"The question I have is since I paid AMT based on the company valuation two years ago, am I allowed now to use the price based on two years ago (instead of my exercise price) to calculate capital gain."
No, not if you're referring to your "regular tax" calculation of gain. With a Qualified Disposition you use the "out of pocket" purchase price of the stock you exercised and did not sell for regular income tax purposes. That's really the whole advantage of selling stock acquired via an ISO: your gain is taxed at low long term gain rates when you sell instead of having the "built in" gain taxed at ordinary tax rates when you exercise, as with an NQSO.
For AMT purposes you'll use the higher "fair market value" basis, resulting in a much lower capital gain. Based on the mechanics of the AMT vs. regular tax comparison you might get some or all of the AMT credit.
As you know, each year you're supposed to do two tax calculations; one using regular tax rules and another using the AMT rules. Then you pay the higher of the two. So you can't think of the AMT "penalty" as purely a "timing difference", i.e., I'll pay a boatload of tax when I exercise and hold but I'll get it all back when I sell." That would sort of defeat the purpose of the AMT.
The AMT rules have changed quite a bit with the 2018 tax law changes and I can't claim to be an expert here. I think talking to a CPA who deals with high net worth individuals would be the best way to get a good understanding of what's going on.
