Adjusting percentage values on cell change.

Good afternoon Gentlemen, I have a column of data, with a varying number of percentage values that add up to 100%, separated by "NA", i.e.

33%
33%
33%
NA
25%
25%
25%
25%
NA
100%
NA
20%
20%
20%
20%
20%
NA

Now... when I change one of the values I would like the others to even up, i.e. in the last example if I change a 20% to 50% I would like the others to change to 10%... any ideas?