Let your ear be your guide. Mic placement is too complex for simple rules, and is heavily influenced by the instrument and the room. Even small position changes can make big differences - only a 4" different location can sound like a totally different EQ. Put on good headphones and listen carefully to the sound as you try the mic in different locations. Or make recordings and notes of various microphone positions. You'll eventually come up with a spot that you like. I have not recorded my X20 so generalities are all I can offer. There are many interactions going on in and around the sound hole - don't point it there.
An SM58 is a decent starting place, but a condenser mic will likely give you more pleasing tone and an accurate representation of acoustic guitar. As a stage hand, I've had plenty of trouble with performers - even pro's - at festivals. I carefully explain that the best balanced acoustic sound is pointing the mic at the 14th fret from about 30° off axis and roughly a foot away. And then they immediately move to put the mic directly in front of the sound hole at about 6". Drove our sound tech nuts as she spun off the bass frequencies to control boominess.
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