The article by Shane in this years Powder gear guide said it like this: For a full rockered ski, go 10 cm longer than normal, for a half rockered ski, go 5 cm longer, and for mini rocker stay the same length. So for Obsetheds (tip rocker) id say go at least 5 cm longer than normal. So if you usually ski a 161, a properly mounted 169 seth would probably be perfect.