Working with forms can be tricky. One thing you should always check is whether the calculations are being done in the correct order. For example, you usually need a subtotal before you can calculate tax, and you need those values before you can calculate a final total.
Hidden away in the Form tools under the More heading is the Set Field Calculation Order command. Click it to open the dialog shown here. Review the list and change the order if necessary.
Would you like to try it out? Here’s a little order form with some calculations out of whack.