It turns out that although one cannot override Line 18 of the IRA Info Wks directly (i.e. via Ctrl-D), one can do so by providing supporting details (i.e. via Ctrl-I). Entering information in the details that changes the final result is accepted.
Whether or not it will break electronic filing, I don't know yet (over-rides often do). I will update this when I file.
The weird thing is, this value is really important if your IRA that had non-qualifying contributions (i.e. if at least one contribution to the IRA was NOT deducted on that year's taxes). Once entered, the calculation seems correct (it affects the amount of an IRA distribution that is taxed). However, TTax is not correctly capturing this number, even when it is queried for and provided.
