The last column is the ratio between column b and a, the last row is the
sum of
each row, the second formula only applies from the second row to the row
before
last row. The table that doesn't work is:

    |     |      a |     b |      ratio |
    |     |  0.797 | 0.780 |            |
    |     |  0.744 | 0.730 |            |
    |     | 12.707 | 2.384 |            |
    | sum | 14.248 | 3.114 | 0.92040988 |
    | ^   |      x |     y |            |
    #+tblfm: $x=vsum(@2..@-1) :: $y=vsum(@3..@-1)
    #+tblfm: $4 = ($3 / $2) * 100