Withholding On sales tax at rate 20%

Can any one guide me how can i set withholding 20% on sale tax
sale tax 13%

Does this help? Account for withholding tax on sales invoices | Manager

this is a feature still in #ideas waiting to be implemented.

no i know this feature it works on net amount ,
i wanna withhold on sale tax amount
like contract amount is 527859 * sale tax 13%= 68620
total sale tax invoice amount is 527859+68620=582,746.00
Payment Method in pakistan
582746income tax withholding 3%=Rs. 17,482.38 & 20% of sale tax amount which is 6862020%=13724
so the chq amount is 582746-17482.38-13724=564,917.86
13724 calulation system is missing in manager io

Make the withholding tax % = 5.3% and not 3% 5.30088% to be more accurate)

Net 527,859.00
Sales tax 13% 68,621.67
Gross 596,480.67
Withholding tax 1 3% 17,894.42
Withholding tax 2 20% 13,724.33
Total 5.30% 31,618.75
Balance due 564,861.92

The guide also suggests that in some cases, you may have to calculate the tax manually and then apply it as an amount and not use the rate options