50% profit on insitu tonnes is pretty aggressive, just a couple of factors you are missing.
Resource Recovery - Not a real term, but something that is used to evaluate the affective design of a reserve typically 85%-90% of resource. 10%-15% are tonnes that could not be designed for technical or economic reasons. This is actually missing form the FS but that is something a lot of juniors do.
Internal dilution- Waste or uneconomic sulfide material inside required to be mined with economic material. Typically this is between 10%-15%, really dependent on geologic complexity. This dilutes the head grade. I believe this was also missing in the FS.
Mining recovery - Unplanned material left behind as a result of under break and mucking, industry standard on a new long hole mine would be in the 97% range.
External dilution- Unplanned over break, uneconomic material that dilutes grade< with the strong host rock should probably be in th5% range
Metallurgical recovery - Don’t remember what the FS said and if it was based on bench tests (OK) or a pilot plant (Good) but would probably be 90%-95% for Ni and in the high 70's or low 80's for the precious metals
Freight - Cost per tonne to ship conc. These terms are estimated in the FS cant remember what they are but probably 200-250 dmt
Treatment charges - Smelting charges - again these are estimated in the FS prob around 50$ per tonne of conc.
Payability - % of metal in conc that is paid by smelter, usually in the 95% region for Ni would be a little lower for the precious metals
The opex on its own will probably be 110-120$/t
Royalties will probablt be 3-5% off the top when its all said and done
I still struggle to understand mining taxes so I can no comment on these
