Hi there, aarondenny.

Allow me to share some information about the catch-up deduction item for the Simple IRA to QuickBooks Online Payroll.

When setting up a contribution for your employees, you'll have the option to set up its maximum amount limit.  At this point, you'll want to review the setup. This way, QuickBook Online payroll will calculate its contribution.

I've attached a screenshot for your reference. 

I'm adding these articles for your reference on how to set up the deductions in QuickBooks Online Payroll:

• Retirement plan deductions/contributions
• Set up voluntary deductions in Online Payroll

Also, this link will show you how to run payroll-related reports: Run payroll reports.

Should you need additional help in managing payroll contribution deductions or completing any task with QuickBooks, you can always find me here. I'm just one comment away.