I should also add that when you do NPV calculations you should...

I should also add that when you do NPV calculations you should either risk adjust the project cashflows (and use the risk free interest rate as the discount rate), or use the raw projected cashflows and use a higher discount rate that captures the risk. Mixing both discounted cashflows and risk adjusted discount rates is not a good idea as it makes identifying the key factors more difficult.

The best way of doing NPVs is calculating out all the possible cashflow paths and then presenting the NPV as range. This is of course a pain to do, but it really help avoid making assumption errors in the calculations.