Please follow this guide:

ezPaycheck: How to add a new payroll deduction and withhold it from each paycheck automatically.
https://www.halfpricesof...oll_deduction.asp#amount 

Step 2.1 add a new by-amount deduction.

Step 2.2 Specify the default amount in employee setup.

For example: if the employee works 40 hours for each pay period, you enter the default amount for 40 hours there.

Step 2.3 When you generate a paycheck, you can edit the amount if the employee works for more or less hours.

Please try it.

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How is the Washington State Worker’s comp entered?

It’s calculated by hours worked