I too use the SP b1 mic and love it. However it is not my favorite for acoustic guitar. I love this mic for vocals! And because of its high SPL (sound pressure level) I get very good results with miking electric guitar amps and saxaphone. It has provided me with the best sax sound I've ever gotten.
For acoustic guitar I love the AKG cb2000 mic (inexpensive at around 200).
Both my mics are omni-directional which I don't believe are the best for recording voice and guitar at the same time but it is all I have at the present time. I do get bleed between the two, but I'm ok with it because I'm expecting to keep both guitar and vocals together anyway.
As far as mic placement for acoustic guitar ... It depends on the guitar, the player and playing style (finger picking vs strumming). Here's what I do.
Have the guitarist get into position.
Put the mics away for a min.
Get on my knees in front of the play (ouch doesn't sound good!)
And put my ear close to the guitar in different spots and listen for the sweet spot.
Then place the mic in the spot I found
Start asjusting input levels.
Some acoustics are much more bassy than others so I may choose the bass roll off if necessarry.
Recommendation: Invest in an inexpensive digital camera. Take pictures of mic placements, store the pics in a folder with a document of the same name (as the picture) and take a few notes. I find this invaluable, specially while working on multiple projects where I'm moving mics around a lot.
Joe C