wrong % discount and Tax(VAT) calculation

Ok, i have a little problem here with Order Edit and Quantity Discounts where my customers get a discount when they reach a surtain amount.

My customers are supposed to get a 5% discount when they reach 5000 kr, the the tax is supposed to be calculated from the discounted amound. Now there is no problem with this untill i need to edit the orders.

What i get when i take out a product is the sub-total turns into amount without tax, second, the Quantity Discount is not calculated again and turns into a + on the total amount, then the tax is added on top, which is wrong since the calculation should be (sub-total - 5%discount + tax). hope anyone can help

Best regards
Gunni