How do I add a formula to a custom field?

You can't. Intuit hasn't made anything like that possible with QuickBooks. Custom fields don't interact with any other fields.

I appreciate you for sharing your thoughts and ideas with us, @TreeBoss.

The option to make a custom field and to show a formula of 30 days before the due date field isn't possible in QuickBooks Desktop (QBDT).

The due date of your invoice depends on the Terms you've selected. I can see having the option to make a custom field with your formula is beneficial to your business. I take note of this as a product suggestion. In the meantime, you can add the Other field of your invoice and name it your preferred field. Then manually enter and calculate the days before the due date. Here's how.

1. Go to the Lists menu and select Templates. 
2. Double click on the type of form you’d like to customize.
3. Select Additional Customization.
4. From the Header column, put a checkmark in the Screen and Print box in the Other field. Then enter a name for that field.  
5. When you’re done, select OK.
6. Select Print Preview to see your form. If everything looks good, select Close then OK. 

You can also read this link for more details about this process: Use and customize form templates.

Additionally, you can also look for a third-party app that lets you create and add a formula to your invoice. Just hover to the App Center and look for an app from there.

When everything is all set, you might want to check out this link for reference in efficiently handling your customer's payments: Record an invoice payment.

In case you have other follow-up questions about modifying your invoices, you're always welcome to add them in your reply. I'll be more than happy to answer them for you. Have a good one and always stay healthy!