Simplest way would be to insert a new column immediately left of the results column. The results would use INDEX() functions that included that 'extra' column. Then, when inserting new monthly columns, you insert to the left of that extra column.
Use this formula in H2 (after inserting that new 'extra' column):

=INDEX(D2:G2,COUNT(D2:G2))-INDEX(D2:G2,COUNT(D2:G2)-1) and drag down