Hello, Construct SB. Glad you've joined us, let me share direct information about adding a line item percentage. 

Currently, QuickBooks Online doesn't have the option to create a line that auto-calculates a fixed percentage for subtotal. You need to enter them manually, for more automated solutions consider using a third-party app that calculates fixed percentages on subtotals.

Alternatively, you can create an item with a fixed percentage for the subtotal, but you will need to manually calculate the rate each time to get the correct figures.

I added this article for you to scan if ever you need to learn more details about how to personalize and add specific info to your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.

For customized support with QuickBooks activities like report management or transaction tracking, discover our QuickBooks Live Expert Assisted option. Our skilled experts are available to help you through every aspect of your QuickBooks experience, guaranteeing you get the best possible results.

Your attention to detail and commitment to accurate record-keeping are truly commendable. If you have further questions about Builder's fee line, or anything else. Please feel free to Reply below!