It shows how to configure a calculation template that works with fixed amounts. So you need to configure related calculation template (or create new one) and set calculation type as Fixed Amount
.
Also check this tutorial. It shows (method 4) how to add a button that calculates discounts from amounts. So instead of entering 5.32 you can enter 100 (target amount) to deduct 5.32 from 105.32
I wanted to show you how search feature works. There are lots of useful information.
PS: Round button located next to Discount % button also does what you want.