One last question that confuses me a bit is the NPV calculations for the IIR. Are the calculations so at 12% and 20% on the individual cash inflows over four years for example or are they an annuity which only works if the flows are even. Is scrap added on too. And which table do you use exactly in the tables.
You discount all the cash flows, including any scrap.
You can always discount each year individually using the present value tables, but if the flows are equal each year then it is quicker to use the annuity tables.