i mounted mine 2 cm's foreward of recommended, I love them for everything, but in retrospect, mounting them right on the line would be just fine.  The recommended point is about 6 cms back from true center, but if you are going to do things in reference to true center, get out the measuring tape and do things that way.  don't mount them any farther foreward than 2cms though, especially if they aren't your park ski, all though the p4's are becoming my favorite jump skis.