***If you wanted to go into more detail, you would adjust the...

***If you wanted to go into more detail, you would adjust the NPV to reflect the likely Operating costs, due to fuel costs which impact the cost of mining, operating the production facility, transport to the port and shipping. You could also add in inflation to the staff costs.
If you look at the Red line in the chart above, it shows the impacts on operating cost, if the operating cost increased by 30% this would reduce the forecast NPV by US$25mil. Which is the same as a 10% reduction in revenue (based on the graphite prices)*** Can you see the error's in your methods of calculating spid.

Would you like me to run the NPV calculation again with a 15% increase in operating costs and the 15% reduction in graphite prices (revenue) or do you understand the chart now? What purposes would this serve spid.

Why don't your run a calculation on current information. 15% increase in Flake Gaphite prises, 23% increase in plant capacity and $1.5M spent on additional plant expansion. As always, DYOR.