Using annuity factors with a delay

I would appreciate your help. It is not uncommon when evaluating a project to have a period of flat expected cash flows as the project stabilises out. For example cash outflows at the beginning, a period of growth in cash inflows for years 1 & 2 and then flat cashflows for year 3-10. When discounting the cashflows it is quicker to use annuities for the period 3-10, adjusting for years 1 & 2. What is the right way to do this?

Should I take the annuity factor for 10 years less the discount factor for 2 years or should I just use the annuity factor for 8 years, being the length of time of the flat period? I’ve seen examples from ACCA for a 1 year delay (so the flat period started in year 2) where the annuity – discount method was used and then an example for a 2 year delay (with the flat period beginning in year 3) where the length of the flat period was used to select the relevant annuity factor to use.

I am confused, when I worked the example myself I used the annuity – discount method. This gave a lower figure for the annuity factor than that used in the example. As this would reduce the PV of the cashflows and could make the project unacceptable I want to be sure I use the right method to calculate the annuity rate as it takes too long to do individual calculations for each year!

I hope this question makes sense, I really appreciate your help with this.