I'm travelling sans laptop to plug it in to my model but it's pretty close to 30% post tax NPV uplift going from 13 to 20 years.
With prices where they are right now I think a sliding price assumption is best. I think anyone producing in 21/22 will get a full 12 months of super pricing until inventories rebalance before a fall to long term average which $13 seems to be a fair proxy for. Won't play out exactly that way but averages work out over time.
A year of $20 pricing will bolt >$150 on to NPV, slash payback period and set the balance sheet up for expansion if conditions warrant.
Does the 16th count as mid to late October?