Simply knowing the basis for each of your mutual fund shares is not, in and of itself, necessarily enough to calculate your gain or loss.
Unless you sell all of your shares in the fund at one time, you also still need to determine exactly which shares you sold.
Identifying the shares sold, in turn, affects the data you'll enter for your basis and holding period when you complete the Capital Gains/Losses section of the Interview (or the Capital Gains and Losses Worksheet if you're using the Forms method).