If you want the two notes of a double-stop to have equal voice, the correct arm level will be the midpoint between the two strings.
Try this in both the upper and lower halves of the bow, as it can take more care to find the best arm level in the lower half.
Holding the screw of the bow is a useful practice technique whenever you suspect the bowhold is causing problems.
Voice the chord
If however you want to voice the double-stop and bring out one line more than the other, you will need make your right arm level closer to the desired string. In this situation also think about point of contact differences. To bring out a melody line, try angling the bow (making the bow stroke crooked) so that the desired string has a point of contact closer to the bridge than the less important line on the other string....