That feature is not available, I know it will surprise you. However after complaining about it Lubos implemented the ability to select the option for manager to calculate Withholding tax on the total sales (VAT inclusive) or select the option that allows the user to manually enter Withholding tax amount. The second option I believe solves your problem and further more, the percentage option calculate Withholding tax on total sales which will be a problem if some invoice items or line items are not withholding tax deductible or the Withholding tax rates are different for the invoice items/ line items. The option of manually entering Withholding tax solves that issue too.
Click the drop down where the Withholding tax field is located to switch between percentages and amounts