I don't think it's even physically possible for me to place the thumb that way on the mandolin (as I've mentioned before, I have very little control over my left thumb, since one tendon is missing. My attitude in both cases is I don't place the thumb at all; I fix the instrument in place, then bring my hand to the neck and let the thumb land where it lands, mostly, in both cases, on the side, perhaps lower on the guitar. Barreing will ofen force my thumb below the fretboard, but is to be avoided anyway. Complete barrre chords don't connect well.
The right hand may be a different issue. Especially on a mandolin without pickguard players tend to brush the strings behind the bridge (for tactile reference, or, perhaps simpy because the strings are there.) That tecjhnique does not translate to a flattop guitar.
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