Electronic tuning devices are best used as generators of aural references, to which the tones of the acoustic instrument being tuned are matched by ear. DO NOT use the meter! Naturally, this precludes the use of any electronic device which does not have tone generating capability.
The best way to listen to the device is with a pair of cheap non-isolating headphones. The ubiquitous earbuds are ideal, provided they are not of the type with a tapered body which can be firmly jammed into the ear. You need to be able to hear the instrument easily while wearing the headphones/earphones.
Beyond that, the device must also produce tone which has a good rich mixture of harmonics. This excludes any device which produces only sine waves. Many also produce a square wave because it is so easy to generate; however, it lacks all the even-numbered harmonics, and therefore only gives you half the aural information you could be using. Try to find a device that generates a sawtooth wave form.
Be careful not to play the sound of the tuner too loud, as it can easily overpower the sound of the instrument, masking the quieter spectral content of the upper harmonics which are absolutely critical to your ability to tune. You can easily find a good volume level which makes the phase cancellation obvious with a little experimentation. You may find you have to adjust it somewhat differently for different registers on the instrument. You may also find that setting the device to an octave or two above or below the note you are trying to tune can help resolve troublesome notes or registers.
I recommend that the electronic aide be used only for setting the temperament octave and that the rest of the instrument be tuned by ear in octaves. The tenor octave is best for this, for two reasons:
(1) The tension here is almost without exception the highest on the whole instrument, meaning any change you make here will have the largest effect upon the overall tuning. if you set the temperament an octave higher, you may well find yourself ruining your work as you proceed down into the tenor.
(2) Because the strings are so long, you have the best proportion between tuning pin rotation and change of pitch, meaning it is easier to be more accurate.