The general rule when working out a reduction in personal allowance is that if your income is greater than £100,000, your allowance will be reduced by £1 for every £2 over £100,000.
For example, if your income is £110,000, your personal allowance will be reduced by £5,000.
The £100,000 limit is extended by the gross amount of any gift aid or charity contributions that you make. If you make £2,000 of gross contributions (£1,600 net), the limit will increase to £102,000. Your personal allowance will then be reduced by £1 for every £2 over £102,000.