Add Payment amount to SO Invoice Form
Unfortunately there are some fields that only calculate in the screen and this is one of them. I'm not 100% sure, but I think the reason for it has to do with performance concerns.
It should be a pretty light customization for them to enable this field in the DAC to calculate globally and not just in the screen.
You might even ask how to do it on StackOverflow and maybe a developer will do it for you.