Seat post extension. How much is too much?

Asked in my CP thread but figure I'll get more exposure here.

My Marin has a 370mm post fitted, 130mm in the frame and 240mm out of it.

Does this sound ok?

The min insert mark is inside the frame, probably 10-15mm below the end if the seat tube.

I'm probably only worrying about this because I read on retrobike about someone that had bent the seat tube (not post) on their bike probably by having an over extended seatpost. Think that might have been an ali frame though so I probably have nothing to worry about?

Frame is Columbus Squadra Corse tubing and the fit is good otherwise so don't just tell me to get a bigger frame.

The bottom of the post needs to be inserted so that it extends about 1cm below the lower edge of the top tube or to the min insert line (which usually means even deeper)

I'm not sure how anyone could bend a seat tube and not the post first if it's correctly sized and fitted .. simply the wall thickness of the average seat post would suggest it would bend first. Although I'm sure a someone will be along in a moment to prove me wrong with some maths

Walk thickness?

My seatposts are all thicker walled than the seat tubes they live in.

I guess that the case I read about on retrobike must have been an older frame and maybe a lightweight thing or something.

Just seen this.

at least a quarter of the length of the post in the frame is my rule of thumb