The field at the top of the editor is intended for your tax base formula. The formula field is not manually editable - to enter your formula into this field, you will need to provide components for the formula using the fields and buttons below. Your formula will be assembled from the components you provide.

It is possible to provide the following components:

Mathematical signs are used to indicate the operations that need to be performed on the other components of the formula. They can be added to the formula  by clicking the appropriate buttons: [ + ] for addition, [ - ] for subtraction, [ * ] for multiplication, and [ / ] for division.

Variables can be selected from the drop-down box below the row of buttons with mathematical signs. In the list of variables that can be selected, ST stands for 'Subtotal amount', DST - for 'Discounted subtotal amount', SH - for 'Shipping cost'. Taxes are labeled by their own names. To add a variable, select it from the drop-down box and click the Add button next to it.

Numbers can be added using the input field located below the drop-down box with the list of available variables. To add a number, simply type it into the input field and click the Add button on the right-hand side of the field.

For example, if you want your tax to be applied to the sum of discounted order subtotal and shipping cost:

You can cancel the latest action using the Undo button, re-apply the change after the cancellation by clicking the Redo button, and clear the formula field by clicking the Clear button.