I asked the same question on the Evans Cycles website.
Answer given is below:-
Answer:
I’ve taken a few measurements for you from the 380mm post that you may find useful:
The 380mm Reverb post measures aprox 375mm from the base of the post to the centre of the saddle rails.
Subtract the 80mm minimum insertion of the post that must be kept inside your frame (or whatever your frame manufacturer says should be the minimum seatpost insertion in their frame, whichever is greater) which leaves you with aprox 295mm max height, from where the post comes out of the frame to the centre of the saddle rails.
Regarding other sizes, the measurement from the base of the post to the centre of the saddle rail is always aprox 5mm shorter than the complete post length for all size Reverbs, and all have an 80mm minimum insertion (unless your frame manufacturer states more for their particular frame, use whichever is greater)
Hopefully this helps??
Incidently, I am 5ft 9in, and went for the 380mm post with 125mm drop and that is working great!!!