SLA to derived WHT account from expense line item

Hi,

We have a scenario where we need to derived one segment of the Withholding Tax (WHT) account from item expense account.

for example:

Dr. Item Expense  1611-00006-0008-5510023-0000-00000-0000-000            

Cr.  Liability                           1611-00000-0000-2722855-0000-00000-0000-000

Cr.  WHT                               1611-00000-0000-2722631-0103-00000-0000-000 --> segment 3 of the WHT should be same as segment 3 from item expense (in this case 0008)

We try to create a custom source in SLA to change the WHT account derivation rule (see file attached for detail). But, the WHT account is still using the default account from tax setup, not derived from item expense.