Originally Posted by DaveSSS

The best rule is to be sure that the post extends at least to the lower edge of the TT. A full centimeter below is better. That rule has been around for a very long time. I wouldn't go less than 5cm.

My LOOK 585 frames have a reduced diameter area at the top to reduce the seat tube diameter down to 27.2. The total length is 6cm. Any amount of post lower than that is just hanging in mid air.