In the lessons, one of the first thing suggested is to tune the guitar. I have 4 different electric tuners and 4 different settings. Have a Snark clip-on tuner, Rocksmith tuner, built in tuner for amp (Fender Musting I V2), & phone application tuner.

All of these seem to give slightly different tunes. I have been just using the Rocksmith tuner because that is forced during that play time. Is there any way to test the accuracy of any of these tuners? They seem non-adjustable.