"GetTaxed Subtotal" is not valid, so that wont work
do you have the option in the tax settings to include shipping in the tax?
you will need to uncheck that option in order to use the taxable total in the shipping.
if you set the tax to include the shipping, and use the taxable total in the shipping it causes an endless loop.
you could get the total minus discounts using:
(WA_eCart_TotalColumn(eCart1, "TotalPrice") - WA_eCart_GetDiscounts(eCart1))