Hi Guest, Unfortunately, due to the nature of the beast there will always be a margin of error no matter what method you use. Open and fretted at the twelth is probably the most favoured, or the harmonics route you mentioned. Some use the harmonic way as the string is not geting stretched in any way by being pressed down. I use this route myself but each to their own. If you really want to split hairs then go to the 19th fret...whatever you do however, will have marginal errors......its as certain as death and taxes.
Fenderphil
Trust me...look at my pic.....Just kidding.
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