Thank you for getting back to us, @alex-pearce.
You're right. You'll need to set up your deduction item as pre-tax. Let me provide some details.
Setting up your deduction item as Other after tax deduction means that your deductions are taken out from your employee's net pay. And setting it up as a Pre-tax deduction denotes that deductions are taken out from the gross pay.
Also, if you set up your deduction as pre-taxed, you won't be taxed twice on your paychecks.
You may follow the steps given by my colleague above in setting up a deduction.
Fill me in if you have any other concerns.