dropper post

spr wheelie

just got some money from santa and want to buy a dropper post.

been looking mostly at a reverb the ks lev if i can find one for less than two hundred.

my main question is what size do i need.

the bike is a norco sight 3 se im pretty sure the seatpost diamater is 30.9
my current seat post is roughly 350mm
when its down as low as it can go is rougly 90mm showing which is perfect
when im peddling on road there is rougly 220mm showing.

so im reckoning i need the 125mm adjustibility but what overall post length would i need.

also as a side is there any other posts it would be worthwile considering.

cheers