Let's say you've got 5 percentages. Sort them, from high to low, then make the smallest one 100-(sum of the 4 bigger ones). This forces them to add up the way you want, but it pushes all the round off error into the smallest percentage. Another way is to make the largest value 100-(sum of the 4 smallest). This pushes the error into the largest value. Neither way is "correct" in a strict mathematical sense, but I'm assuming that's not much of a priority for you anyway.