I don’t know what type of uni you have but i do have plenty of experience
in the custom seat post area. I currently own three seatposts (for my
miyatas) which i had tom miller build and they range in height from 16.5"
for the first one to 18.5" for the most recent. I don’t know what my inseam
is because i have never measured it that way. I always take the measurment
from where the seat tube ends at the bottom of the fork crown up to the
bottom of the seat base plate and then i add the 1/8" for the base plate.
(since i don’t want to outgrow an expensive seat post i measure for the
absolute max amount of tubing that my uni can accomidate and still fit me.)
Of course the way i measure it my seat post cant be lowered at all, just
raised and with a standard 20" miyata frame (or a probably any standard 20"
frame) the absolute maximum variance you could have in height is about 5",
and that’s including the extra inch that the seatpost can hang out of the
bottom of the seat tube without touching the tire.
In summary: I am about 6' and for someone aboput my height, you'de need
between 16-19" of height from the bottom of the fork crown and the bottom
of the seat. That's just an estimate since everyones proportions are
different and also since uni's vary you'll need to adjust your measurments
for your particular cycle. Good luck.
(my new account doesn’t yet have a signature) Karl Frankowski