They want to know your estimate of what you are going to earn in the whole tax year - so you can never change an estimate from 11,000 to 0 within a financial year unless you didn't actually earn the 11,000 - IYKWIM. So if you have earned 11,000 this financial year then you leave it at 11,000 until next financial year, when you will submit a new estimate of what you think you will earn next financial year.