I have a 29r steel frame mtn. bike. Seatpost diameter is 27.2. To get the proper saddle height, I have to extend the stock 350mm easton seatpost about 2.5cm beyond the min. insertion line. This leaves ~8.5cm inserted into the frame, well below the top tube intersection. The specs for a thomson elite 410mm seatpost list a min. insert depth of 7.5cm. So, at 8.5cm inserted, am I okay, even though it is above the min. insert line? I was surprised the 2 manufacturers had min. insert lines so different.

That surprises me too. Almost every seatpost I've ever measured, including long MTB types, have the minimum insertion mark about 7.5 -8.0 cm above the lower end. Having the end below the toptube/seattube intersection is important too but you report you are OK there.

I just measured several Easton and Trek house brand seatposts in my parts box in both 27.2 and 31.6 mm. All are MTB length and all have the minimum insertion 75 - 80 mm above the end. The one odd exception is an Easton EA30 (their cheapest, heaviest model) in 31.6 mm and the minimum insertion is 100 mm above the end.

maybe it's an easton thing? Just measured my easton, it's 100mm also.

There are many different minimum insertion length that range from 7-10cm. Thre's no way to know what's driving those differences.

I have three frames that all have 27.2mm reducing bushings built into the frame, that are no more than 6cm in length. From an insertion standpoint, anything over 6cm just hangs in the open space below the bushing and does nothing.

The other side of the issue is maximum extension. If you can see 2.5cm of the post that's supposed to be inside the frame, then you've gone beyond the maximum extension. Whether that's an issue or not is hard to say. You'll never get a manufacturer to say it's OK, but it may cause no problem. How badly do you abuse your seatpost? Do you slam yourself onto the saddle a lot? That might be a problem with too much extension.

Not all of them. As I mentioned, I measured three Easton seatposts. Two were 75 mm and only one was 100 mm.

I don't ride too hard, I'm too old! I'm going to keep a lookout for a deal on a thomson, but ride mine as is meanwhile.

I'm thinking that sloping top tube bikes might be a driving factor. The seat post manufacturer doesn't know what kind of bike it's going into so they're going to want to be covered for the worst case.