Overriding line 6 is an improper way to do this, particularly when there is a correct way that's just as easy. If you are able to do an override of line 6, you are apparently using the CD/download version, not the online version indicated in the tag on the original question, so instead of overriding, in forms mode you would enter the year-end balance on line 18 of the IRA Information Worksheet and it will then propagate to line 6 of the Form 8606. Line 18 of the IRA Information Worksheet is is where TurboTax would put your year-end balance if entered it in step-by-step mode.
