Warn user when bucket percentages add up to less than 100%

[amdw]

Currently the percentages go red when they add up to more than 100%, but there is no indication when the sum is below 100%, or any convenient way to see how far short we are or "balance" the allocation by choosing a particular bucket to allocate the missing percentage to in order to bring the sum back up to 100%.

This is related to, and would become more important with, #215 but could be done independently.