I recall though (only from memory I might be wrong) that although the deposit is about 250Mt the actual recoverable ore is around 190Mt - I can't remember the exact number.
So I would round that overall NPV down a little for the unrecoverable ore, but otherwise I think your assumptions are spot on and as you say they provide a nice upside if FOB is higher or the % royalty is a little higher than assumed.
IOH Price at posting:
84.5¢ Sentiment: Hold Disclosure: Held