I know that starting from a perfect interval and shortening the interval by two semitones gives a doubly-diminished interval. But what about starting with a major interval?
For instance, starting with a major 6th, I have 9 semitones. If I shorten it to 8 semitones, I have a minor 6th, and 7 semitones is a diminished 6th. Would 6 semitones (3 less than were we started) be a double diminished 6th, or a triply diminished sixth?
In other words, is the "numeric name" of a diminished interval based on how many semitones it is below the "reference" interval (major or perfect), or how many semitones it is starting from singly diminished?