Line 18 of Form 1040X should show the amount of your previous overpayment (presumably the amount refunded). TurboTax will automatically place this amount on line 18 when you begin the amendment. The calculation will proceed through lines 19 and 20 to determine the amount that you owe with the filing of the amendment. The amount you were refunded (line 18) is subtracted from the amount of your original tax payments paid for the year (line 17) and the difference between that result (line 19) and your newly calculated overall tax liability (line 20) is what you now owe.
