|Maybe something like this will get you started:|
In A1 I entered a value as the starting weight.
In B1 I put this formula and formatted it as a percentage:
As I enter a new weight in A2, then A3, then A4, the percentage shown in B1 will change.
The INDIRECT function determines the cell to be used in the calculation by counting the number on entries in Column A, which in essence gives you the Row number of the last entry.
For example, with the last entry in A4, there will be 4 pieces of data in Column A and the formula will essentially evaluate to:
You just can't have any blanks in the list. The formula can be modified to fit other layouts, but we'd need to know the exact location of your data before we could make any changes.