AOS discount: need help (or missing feature?)

I’m trying to produce a quote (PDF) in AOS that has the following values:

list price net = $2000
vat 8%

list price net = $3000
vat 8%

list price net = $5000
vat 8%

total amount net = $10’000
total discount = -$500
subtotal = $9500
total tax = $760 (8% of subtotal)
Grand Total = $10’260

I can’t find a way to set up my line items giving me the correct values for total, total discount, tax amount and Grand total. I’ve gotten close adding a hidden discount to a line-item (not showing it in PDF), but it always screws something up, especially the global subtotal ($9500 in my example).

Any ideas how I could get these values using AOS arithmetics?
Is this simply not possible the way AOS is set up?