Allow specifying a created or resized partition size as A) a percentage of available unallocated space; or B) an 'x' multiple of the 'used size' of the currently selected partition.

An example: a two-year-old 18GB Graphics partition has 14GB of data. If I select the partition for Resize and specify a Used Size multiplier of x2 the resulting partition would be 28GB, which might reasonably provide room for another two years of accumulated files... or x2.5 for a 35GB partition, etc.