Work out 90% and 50% of gross pay. Start a column for deductions from each. 1) Work out TPS deductions (% of gross salary). Your existing payslips will allow you to work out the percentage deduction (around 7%) 2)Work out NI. Inland revenue site gives the % to use depending on whether you are in an occupational pension (Contracted out NI) or not in TPS and thus paying higher NI. It also tells you how much of monthly salary is NI exempt (about £400 per month). 3) Work out tax. You need to look at your Tax Code and divide it by 12. 747 Code means £7470 per year of income is tax free. Divide that by 12 for the monthy tax free element. You deduct the tax free pay from the pay left after pension deductions and you work out 20% of that figure for your monthly tax bill. 4) If you repay student loans, you ignore the first £1250 per month of pay and work out 9% of the remainder.