Hi @warren6,

Let's set up your Discount value field on your invoice placed after VAT so you'll get the correct total amount.

Here's how:

1. Go to the + New menu.
2. Select Invoice.
3. Fill up the form as necessary. 
4. Enter the discount amount on the Discount value field. 
5. Press the Switch (down and up arrow) button, so your VAT gets deducted from the subtotal before the discount. 
6. Hit Save and close.

Take a look at this article to learn more about how different accounting methods affect sales tax: How Cash and Accrual accounting affect sales tax. 

Have other questions in mind? Post them in the comments below, and I'll get back to you.