tuning with a chromatic tuner

my chromatic tuner has an adjustable setting for 440hz up or down. my band plays 1/2 step flat (for the singers voice). i can't read the tuner at some of the gigs due to lousy stage lighting, and have resorted to dropping the tuners setting down from 440 to like 420, so that the green light comes on when the tuning is right (flat).
but my question is, is 420 the right 1/2 step flat setting. i arrived at 420 by setting the bass up on another tuner that had a setting for 1/2 step flat tuning and trying to match it..

Doing the experiment with my tuners I get 415hz (in fact, that is a minute amount flat - as one would expect).

Or that the other tuner with the half step is accurate?

The answer to your question though, depends on what ever the rest of the band are tuning too. When the whole band is tuning up or down to accommodate singers/bagpipes/whatever it is important that *everyone* agrees on the calibration and uses the same figure to calibrate their own tuners - or tunes with the same tuner - otherwise you'll be in tuning hell. It doesn't really matter if that is an exact half step or not - what is important is that you all calibrate to the same value - having said that in our band tuning up to A445 is ok, but tuning to A446 to A448 - even when everyone is absolutely correctly tuned using identical tuners - somehow just sounds too weird. So we now tune to
A445 regardless of what pitch the bagpipes are at.

At your next practise compare all your tuners - and decide on a calibration interval - stick to that and tape it onto the tuners if you have to. If any of you play in other bands as well, or retune to A440 for practising at home, make a point of reminding *everyone* to calibrate to (whatever) and retune before every gig.

If you have a keyboard player make sure they're tuning to the same note
- some keyboards calibrate in hz, and some in cents, it can get confusing.