No ... do not override the calculated amount by entering a zero in box 2a ... doing so will get you an IRS letter later. As I mentioned above ... unless you know of a reason that the distribution should not be fully or partially taxed it is fully taxable. 99% of the time it is fully taxable even if box 2a is blank or they write in unknown.
Box 2b should be checked and the program will walk you thru the following screens to determine if any of the amount is not taxable.
