You can use Purchase Tax Rate for this purpose. The purchase tax is paid by the company on the expenses and is subtracted from the charge amount to avoid double-taxation for the client. Core uses this rate to separate the actual cost and tax amount. If you create a vendor bill with a purchase tax on it, Core deducts that tax from its income account. If you enter an amount instead of a percentage, Core reverse calculates the percentage value.
Cost Amount = Net Cost Amount + Purchase Tax Rate
Cost with Purchase Tax = Amount without Purchase Tax / (1 + Purchase Tax Rate / 100)
For example, If you apply a purchase tax of 10% to a vendor bill of $100, Core recalculates the expense amount to $96.36. To show the Purchase Tax Rate column on the Edit Vendor Bill screen, go to More > Show/ Hide Columns > Purchase Tax Rate.