Selfie

I am technically self employed in construction, work through CIS, therefore work is sourced by agency and pay provided via agency's selected payroll company. Payroll company charge me a fee each week and I get confused which amount I count for my gross income. Please see example of my weekly payslip: Contracted amount/gross income: 500 Payroll fee: 20 Subtotal: 480 Tax taken: (20% CIS): 96 Net pay: 384 When I record my income for tax purposes, would I need to state my contracted amount: 500 or sub total gross (without agency fee): 480. Because if I account 500 as my gross, then add agency fee as expense, this expense position does not contribute in reducing my income tax at all..because HMRC will account that I had to pay tax from 500, which I did not do..as I paid tax from subtotal or 480, even my initial contracted amount (and it appears on payslip) was 500. I would appreciate expert explanation what is correct way to account for my income in self assessment.