It doesn't matter which holes at the dropout you use. I wouldn't use a mono stay adapter on anyting other than a mono stay, instead I'd replace the seatpost clamp with one that has rack mounting holes integrated (I prefer this method even with a mono stay). If the QR feature is important, Bontrager makes a clamp with a QR http://bontrager.com/model/08427 all other clamps i've seen use a bolt.