NPV & sensativity analysis

Hello everyone , just want to confirm several things : 1. with NPV calculation sometimes in the questions the examiner has put : a) fixed costs either to be inflated or not or b) incremental fixed costs Please confirm that I should not take into account the constant fixed costs which are not be inflated as they are not relevant , i.e. not cash flow, incremental and future ? Inflated and incremental fixed costs need to be included into operating cash flow. Also there was R&D costs incurred which should be ingnored in my view . c) working capital I looked to the examiner answer of one exam question related to working capital. Mashine is not to be replaced after the end of project (4 yrs) . The examiner answer is really complicated . Can I just put let say (1,000) as working capital in yr 1 , then put 1,000 in yr 4 to get zero as the masine is not replacable and do not bother with the cumulative claculations for year 2, 3 ? 2. with the sensativity margin calculation a) if we need to find out the sensativity margin related to sales why we should take into account after tax figure of sale ? Let's say NPV is 100, PV of sales before tax 1,000 and after tax 700 (1,000-300 tax) should we claculate sensativity margin as 100/700= or 100/1,000 ? b) if the examiner asks to calculate the sensativity margin related to variable costs should be also make after tax ? As usual thanks in advance to everyone for your input.