two new Extended Withholding tax calculation on net amount with zero VAT

Hi ,

I have a requirement to create 2 new extended WHT codes and types which are to be calculated on vendor amount in invoice postings.Please find below example of the requirement.

W2& W4 are existing and W6 and W7 are existing.

W2- 20%- base amount- 100=80

W4-3.6%- base amount-100=3.6 (total= 76.40)

W6-13.33%- base amount-76.40=10.81

W7-6.95%- base amount- 76.40=5.31

below is the order of line item.

1. Vendor amount 60.91

2. GL amount 100

3. W2 amount -20

4. W4 amount -3.6

5. W6 amount -10.81

6. W7 amount -5.31

I have approached this solution by configuring the tax % itself in W6 as 10.81 instead of 13.33 and in W7 as 5.31 instead of 6.95.

Is there any other approach for it or can we achieve it in standard SAP or do we need to customize it.