Let me add more details to clear up your confusion about the calculation, @Rick. 

I understand that knowing the exact percentage of how much can be deducted per item is important. I'd be happy to provide a sample calculation to guide you appropriately.

If there is only one item on the invoice or no classes are assigned, the payments will be automatically subtracted from the overall sum. But if there are multiple items with different classes, the percentage that can be deducted per item depends on how much you partial. 

Based on your example, the $1150.22 (partial payment) is 1.53% of $74921.33. That said, QuickBooks will use 1.53% per line item. See the sample calculation below for more understanding. 

$1150.22 / 74921.33 * 100 = 1.53% 

$1040.05 / $67745.30 * 100 = 1.53%

$5.60 / $365.00 * 100 = 1.53

$104.57 / $6811.03 * 100 = 1.53 %

Moreover, check out these articles below on how to customize reports by class in QuickBooks and know the sample classes by industry: 

• Filter, sort, or total reports by Class
• Set up and use class tracking in QuickBooks Desktop

This thread is open if you still need clarification about calculating the amounts in QuickBooks. I'm happy to explain this to you further and provide more examples. Keep safe.