Road Cycling - Seatpost length

I am looking at seatposts and see that they range in length from 240mm to 420mm+...So my question is how much length do you need to keep in the seat tube at minimum? Is there some kind of rule? Currently, I have about 130mm above the seatpost clamp which would leave 110mm left inside if I had a 240mm seatpost. Doesn't seem like much; any thoughts?

Generally, longer posts are required for Mtb frames and that's how they're usually differentiated. As long as you're within the insertion limit mark, there's nothing to worry about....

The rule of thumb I have heard/read is that the minimum amount of seatpost needs to extend just below the bottom of the top-tube/seat-tube intersection. This is also what Easton recommends (http://www.eastonbike.com/downloadable_files/instal_instr/seatpost-EN.pdf). My Easton posts also have both a minimum and maximum insert line. This is mainly governed by the section of the stress relief area on the post however.

Good link; thanks.