problem with module quantity discount, tax, and total calucation

i have quantity discount installed, but i met the problem when checkout

• i check out 2 items (price 20 + 30), total shipping cost 15.90, quantity discount 5%, tax 19.6%,
• i set the price with tax, and shipping cost no tax.
• i set the sort order which i want in module:
  sub-total -- 200
  shipping cost -- 210
  quantity discount -- 220
  taxes -- 230
  total -- 240

which i should get is :

sub-total: 50.00
shipping cost: 15.90
quantity discount: -2.50
tax: 7.78
total: 63.40 [ total = 50+15.9-2.5 = 63.40 ]

But, no matter how i set up, its not what i want:

i set quantity discount: include tax - true, re-calculate tax - standard, i get

sub-total: 50.00
shipping cost: 15.90
quantity discount: -2.50 [ 2 items 41.81*0.05=2.09 (include tax = 2.50) ]
tax: 7.70
total: 62.91

i set quantity discount: include tax - true, re-calculate tax - none, i get

sub-total: 50.00
shipping cost: 15.90
quantity discount: -2.50 [ 2 items 41.81*0.05=2.09 (include tax = 2.50) ]
tax: 8.19
total: 63.40

i set quantity discount: include tax - false, re-calculate tax - none, i get

sub-total: 50.00
shipping cost: 15.90
quantity discount: -2.09 [ 2 items 41.81*0.05=2.09 ]
tax: 8.19
total: 63.81

i set quantity discount: include tax - false, re-calculate tax - standard, i get

sub-total: 50.00
shipping cost: 15.90
quantity discount: -2.09 [ 2 items 41.81*0.05=2.09 ]
tax: 7.78
total: 63.40

if i add more items, the amount would be more outrageous

i'm using 1.38a

how do i fix the problem ? thanks :unsure: