You have to pay AMT on the difference between your strike price and the current 409a price of common. It doesn't have much to do with where preferred is valued, other than the fact that a high preferred value, means that the updated 409a common might be higher.
So let us say, your strike price is $0.50 and you are fully vested after 4 years. The company recently does a round where preferred is at $3.00 per share and the new 409a is $1.25/share. When you leave this is what will happen - You need to pay the company $0.50 * number of options you have - You have to pay AMT on (1.25-0.5) * number of shares
With last year's tax law, one good thing is that AMT rules changed, so AMT might not apply. You should, of course, talk to your accountant/tax professional for the proper advice :-)