I play mostly in Csus2 (CGCGCD) and Orkney (CGDGCD).
With the a string (fifth) down to g, an octave below the (third) g string, I listen for the fifth interval between the fifth and sixth strings. I then use the sixth string (now tuned to C) to get the second (b) string up to C. Expect a set of 12-54s to flap a bit and tuning of the sixth string to be off slightly after the fifth fret. You might need to tune the c very, very, very slightly flat to compensate.
Hope this helps,