Not sure if this will be helpful or just confusing, but similarly you can quickly find the notes of any major scale using the Circle of Fifths.
Note that the 6th interval for the relative minor will always be at a 90 degree right angle (clockwise) to the Major root chord. So for Eb major, it's relative minor is C. Or to find another...for B major, it's relative minor is Ab. And obviously, if you're starting with the minor, to find the relative major, go counterclockwise 90 degrees.