Hi. My name is*****'m looking at your question now and will post my answer or ask for more information here in a short while.
Take a look here for information on how to calculate the figure you need. It is called adjusted net income.
Yoru adjusted net income is comprised of all your taxable gross income whether tax has been deducetd at source or not. That includes taxable employment benefits such as a company car and private health insurance. Deduct from the total gross income the gross equivalent of pension contributions paid net of basic rate tax relief, pension contributions pad gross and the gross equivalent of gift aid donations paid net of basic rate tax relief. THat will give you your adjusted net income for the purpose of determining your entitlement to the personal allowance.
I hoe this helps but let me know if you have any further questions.