But if the lowest note is not a C? As we have already seen in the MDS of E and Bb, the system doesn't change. Suppose that we are looking for the interval between E and G. First of all, lets the MDS of E, that, as we have already seen, is expressed like this:

To find the minor interval you need to lower the major interval a half-step. F# minus a half-step becomes F, G# minus a half-step becomes G, etc. Therefore E - G forms a third minor interval (and the two notes are physically seperated by a step and a half). The diminished interval is obtained by removing a half-step from the perfect interval or its minor. We then will find for instance E - A (fourth perfect) or E - Ab (fourth diminished); E - D (seventh minor) or E - Db (seventh diminished). Finally, the aumented interval is obtained by adding a half-step to the perfect interval or major: E - F# (major second) or E - F## (aumented second); E - B (fifth perfect) or E - B# (aumented fifth). Below is the table of the intervals with a base of E.

Lets look at the scale of Ab. In the table, the MDS of Ab is in dark blue. Lowering C by a half-step (third major interval), we get Cb, which is an interval of third minor. Lowering Db a half-step (fourth perfect interval), we get Dbb, which is a fourth diminished interval. Raising F a half-step (sixth major interval), we get F#, which is a sixth aumented interval. Below is the complete chart of the Ab scale.

I apologize if I am a little repetitive, but this subject is very important and you shouldn't underestimate it. I strongly suggest that you practice at length all the scales, so that you can quickly recognize the relationship between two notes at any time, even when playing. We will make much use of it.
Therefore go to the exercises on intervals page, where you can practice these important lessons. Practice until you are very confident with the intervals