To add to what member November said, if a particular flour is not in the pull-down menu, or if there is not another flour in the pull-down menu with the same percentage of protein (shown in parentheses in the pull-down menu), you can use the % entry box just below the pull-down menu. Just remember that the A and B boxes correspond to the MassA and MassB numbers. That way you won't get mixed up and reverse the MassA and MassB numbers.
Another thing to note is that since the AT flour you have is most likely bromated, the final blend will have the bromate diluted. So that might affect the performance of the dough as compared with a flour that is totally bromated or a flour that is not bromated at all. Another option if your recipe does not call for a bromated flour is to just use the King Arthur bread flour, which is unbromated (and unbleached) and has a protein content of 12.7%.