[O] Org table: how to reference part of the rows

[Qiang Fang]

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(@address@hidden) :: $y=vsum(@address@hidden)
    #+tblfm: $4 = ($3 / $2) * 100