Originally Posted by vainamoinen

the key is not really the minumum insertion mark. Many seatposts are marked for old style frames where the top of the seat tube lug is right next to the top of the top tube. But I have a compact style frame that has a significant length of seat tube protruding above the top of the top tube, maybe 3 inches or so.

What you want to do is make sure that the bottom of the cut seatpost protrudes below the bottom of the top tube inside the seat tube when clamped at the correct height for you.

This ensures that you get the structural benefit of the top tube and seat stay cluster in resisting the bending imposed by your body weight.