Current seatpost at highest is 240mm from seat collar to saddle rails

my frame has minimum 4 inches (100mm) of insertion, but I like to run a little more.

frame is only reamed to 8 inches (200mm), but I reckon this is more like 180mm when measuring with current seatpost.

So minimum post size would be 240mm + 100mm = 340mm
Max post size would be 240mm + 180mm = 420mm

So looks like I could get away with either a 355, 380 or a 420mm reverb?

380mm looks the best option I think? giving me 140mm (5-1/2inches) of insertion?

Minimum exposed seat post is 190mm from bottom of collar to seat rails. With a normal seat post height of 240mm you’ll have 140mm inserted seat post with the 380mm long reverb

Why not just get the 420mm, just in case (plus use on other bikes). Or am I missing something?

extra weight?

also didnt want to risk the post not fitting all the way down as the current seatpost gets tighter and tighter towards the bottom when fully dropped

Got a 380/125 30.9 on my large 2011 spot.
It will go fully into the frame if required.
I had to “clean out” the internals of the seattube just to make sure of no catching/scoring as discussed in one of the earlier threads, all is now ok.

stupid question but is the bottom fat bit exactly half of the total length if you know what I mean or a bit longer?