#1
I'm thinking about just getting this chromatic tuner but i dont know what the + or -20 means, is it a bit complicated for what im going to use it for?

my jackson is getting delivered today and im pretty sure it'll need a setup, but i usually just tune by ear via youtube video

ive been using my sister's qwik tuner, think its decent enough to get a proper setup done?
http://www.google.com/products/catalog?q=qwik+tune&um=1&ie=UTF-8&cid=7126759015939290777&sa=X&ei=j76DTaisK43ksQOetvzlAQ&ved=0CD4Q8gIwBg#
#2
your sisters will work in a pinch... but a good chromatic tuner is a good start...
Gear:
2003 Fender Standard Strat w/ Texas Specials
2010 EBMM BFR JP6
2012 Babicz Identity Dreadnaught
2015 Gibson Les Paul Traditional SR
Line 6 POD HD500
Peavey XXX 112
Fender Blues Jr
#3
Chromatics are the best.. but I mean, any tuner will work as long as it's accurate.
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#4
Chromatic just means it can read all the notes vs. just the ones in std tuning. There are usually flat and sharp buttons on non-chromatic tuners that allow you to get the other notes as well. The +/-20 is probably a cents figure, and is horrible if that's the accuracy. You want +/- 1 cent.
#5
As fly135 said, you want a tuner with an dead-on accuracy. It's intonation we're talking about after all.
#6
i would just get a korg. i used one when i started lost it then used one like the qwik for bout a year and finally got a new korg to do my intonation with. they're like 10-20 bucks and really useful. also i like to keep a small pedal cable attached to it all the time so tuning is more accurate.