Efficiently filling formulas in an upper triangular table

I have a table that looks like the one below where the letters are category headers, and the numbers are calculated by a formula. Fill works to propagate the formula throughout the table, but it's important for readability that only the upper triangle is filled, which is inefficient the way I do it now: Fill across row 2, select rightmost N-1 cells, fill down to row 3, select rightmost N-2 cells, fill down to row 4, etc.

Is there a more efficient way to do this? Some of the tables are pretty big, and it's a lot of clicking.