Play the A string at the 6th fret and tune down the low E string to an octave below it. Then tune as normal from the fifth fret, since you now have the low E at D#, when you tune the second string(A) down it will now be at G#, and so on.
Alot of people tune this way all the time, and just capo at the first fret when they need standard tuning.
I hope this makes sense...
Jeff
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