Adjusted Net Income - Deducting Private Pension Contributions

I am trying to calculate adjusted net income to confirm eligibility for 30 free hours childcare. I pay the higher rate of tax (40%). I am planning to make a private contribution to my pension of £18000. I know that I will get 20% basic rate tax relief, and can then apply to HMRC for the additional 20% tax relief. Do calculate adjusted net income, do I deduct £18000 * 1.25 from my gross income to account for basic tax relief OR is it £18000 * 1.67 to account for the additional 20% tax relief? Thank you!

Hi is,
If the pension contribution is coming from your net pay it will be the £18000*1.25 calculation to use. 
Thank you.