You have to guesstimate. Everyone that is claiming ftb has to guesstimate every financial year. Assume that you are going back to work at x time (jan if that's your circs) and work out how much you'd earn if you did. Don't forget to add your ppl to your estimate.

And you are gonna have to get used to estimating. Unless you can survive without ftb, you will have to get used to estimating your incomes.