Count until certain percentage reached

I'd like to be able to look at a list of unsorted numbers and count starting from the largest numbers until the number of counted numbers equals a certain percentage of the sum of all numbers in the list.

For example:
Consider a list such as:

20
30
30
20

If I wanted to know the number of items that equals 60%, it would return 2 (the two 30s).

Any ideas?

I am trying to count through a pivot table, so ideally it'd be nice to do so without helper cells.