To tune a mando directly from a guitar (assuming that the guitar is in tune and has good intonation):

G - guitar 3rd string open
D - guitar 2nd string at 3rd fret
A - guitar 1st string at 5th fret
E - guitar 1st string at 12th fret

I never worry too much about getting all the strings within a course on any course-string instrument exactly in tune with each other. Having just the tiniest bit of variance actually creates a fuller sound to my ears.

Bruce