The seatpost is supported by the butting of the seat tube and the top tube, and there should be enough post below the ST/TT junction to push against. Most CF posts require a minimum insertion of 100 mm (4 in) below the top of the seat tube, for alloy posts it is fairly shorter at about 60 mm (2.35 in), but it depends on the tube wall thickness and may vary from one manufacturer to the other.

Do a simple calculation from your seat height and subtract the length of the seat tube and about 50 mm (top of saddle to rail height). This gives you the amount of exposed seatpost. Add 100 mm (or 60 mm) to get the required minimum length for your post. As others have noted, longer is better.