Calculating of Tax

Hi
I have a question about the calculation of Tax (Moms).
Service cost is 80
Tax (Moms) 25% (=20)
Gross cost =100
So far so good, now I also have a deduction (Copoun) of 50%
That means that the Gross amount should be 50.

My settings are as follow:
Disable payment -> Yes
Enable Tax -> Yes
Tax Payment -> 25%
Show Price including Tax -> Yes

In the Chart it shows as follow:
Net Amount -> 80 (correct)
Discount (50%) -> 40 (correct)
Tax (Moms) -> 20 (Incorrect) This should be 10
Gross Amount -> 60 (Incorrect) this should be 50.

The tax should be calculated AFTER the deduction of 50% so the tax shold be 10 instead of 20.
Or to make this more correct the deduction should be done on the gross amount then everything would be correct.
Net price 80
Tax 20
Gross amount 100
Deduction 50
To Pay (gross amount) 50

However I disable tax and put the price including tax everything is ok but I can´t do this because I must show price both with tax and without tax.

Could you please look in to this and make the calculation of coupons and tax correctly