mypalcal,
For your first question the value per tonne I use uses the recoveries, prices, and inferred resource grade used in the BFS which is $25.17/tonne - however it looks like only 90% of this is payable in the BFS (i.e. smelter/refining charges which probably vary by metal).
The main difference I find is moly prices are half what they were in the PFS, recoveries are different and total capex is higher - although I don't think the capex creep was a large issue. I am also a little confused by why the economics are so different from the PFS but I will wait until they file the BFS on sedar before I really compare the two.
Aside from that I am just using the BFS value (after-tax NPV of $67M) and adding some discounted (or undiscounted) value for the inferred.
Also you cannot do 171Mt x $31.4/t (revenue) x 24% and adjust for taxes and discounting. You need to deduct operating costs/G&A of processing these 171Mt - the mining costs are already sunk and included in the BFS.