I don't know the posh tuners, but mine is an old Boss chromatic I used at gigs to tune silently. At home or pub I generally tune by ear (-phones) once I've got the "E" from a tuner. The Boss has a segment dial showing 440 at the top and a needle that indicates in "cents" how sharp or flat the note is. For my PSGs the open "E"s and "B"s (E9th) get tuned 440 and everything else needs to be from 1 to 4 cents (G#s) flat to sound in tune. None of my strings are tuned sharp on that neck.
C6th is a different game, I tune open "C"s to 440 and the rest by ear. Some need to be a tad # and some flat. Never tune the "E" from the other neck because this one is a "C" based tuning and it's "E" has to be the third in that key.
(PS - I know that 440 refers to the note "A" in Hz, but the tuner likes to use that number for all notes)

(PPS - songs with my old band were usually in F, Bb and the occasional nightmare Eb; due to the singer's range. The above tuning held good even for those keys)