For 90 degree cants - We typically held higher on a pectoral or lower on a shoulder to obtain a center mass hit when using barricades / cover and concealment and engaging enemy from 100-200 meters.
100 Meters = Upper Pectoral Hold
125-200 = Shoulder Hold
For a 45 degree cant, effects are halved.
For targets beyond 150-200 meters we typically assumed a better shooting position.
We didn't bother with any cant less than that as effects were negligible at the above ranges.
This is easily proven on a flat square range shooting groups with different cants and distances.
Mike Pannone's M4 Handbook probably has something about this as well, I'll dig through my copy when I get home.