New seatpost. Told to cut it down.
What’s a general rule for length/minimum insertion length?
Also out of interest why cut it at all? Does it get stuck further down the pillar?
Were you given a reason why to cut it? If it inserts easily to the height you want it, don’t cut it.
Minimum insertion is either the line on the post (usually about 6-8cm) or an inch below the top tube intersection.
The more post you have in the seat tube, the more spread out the force will be.
I don’t see much point in cutting the post unless for some reason it won’t fit further down in the seat tube or your attempting to put your bike on a serious diet :?
There is really no reason to cut your seatpost unless you cannot push it down further enought becuase it is hitting a biddon cage mount hole. You should backhand pimp slap whoever told you those lies.
it might save weight to cut it. these days most seatposts will come pretty long and if it’s a frame with a horizontal top tube you might find that you end up with half the seat tube buried in the seat post. it might save weight to cut that off, but at the same time it’s a negligible amount, for most applications, and leaving it longer gives you more versatility and future choice in migrating it to another frame.
or selling/swapping it when the time comes - nobody is going to want it if you’ve been at it with a hacksaw.
If I have one of those cheap $15 steel seatposts I cut them down a bit (not ridiculously short) saves a bit of weight and I don’t want 400mm worth of seatpost anyway. If I have an expensive post like a thomson I never cut them 
Yay! Less work.