Here's my best guess at this point. Is this right, experts? @Critter-3? @MaryK4 ?
For 1040X Line 6 column A, I'll copy the figure from Line 47 of 2016 1040, which is the sum of Lines 44 (Tax), 45 (AMT), and 46 (Excess advance premium tax credit payment). This seems most consistent with the 1040X instructions quoted above, which I will also follow in filling in 1040X Line 6 column C.
