To record a simple unplugged singer-songwriter setup with a single microphone (I have an SM58) is there specific position and orientation to use for the microphone? Obviously it may need tweaking to get the right balance of guitar/vocal volume depending on the playing and singing loudness but what should my starting point be?
This is actually quite tricky to get perfect, as you have very different tonal characteristics, so all usual guidance is to use two mics, or a mic and a pickup.
But if you have to, you can do it - because of course you can sing to an audience with an acoustic guitar accompaniment.
The challenge is to work out for you what ends up loudest on the mix. If you are standing, i'd tend to move the microphone further from your mouth than vocal mic would be, lower it slightly, and point it at your chest. An SM58 can easily cope with this.
Your tweaking will consist of angling the mic up or down to alter the balance between your voice and your guitar.
But you will want to get yourself another mic or pickup sooner rather than later, then you can EQ them separately, which will really help your recording.
With a single mic, I'd put the mic at about (sitting) head height, and point it somewhat downwards of the singer from, say, 4ft or 1.3m away.
But not with an SM58. That's a singer's mic intended to be used close to the mouth. A mixed singing/acoustic guitar performance with a single mic in that arrangement would rather ask for a small-diaphragm condenser without close-up effect. Sort of tricky.
You have more leeway to get things to sound nice with two mics.
There are microphones that have a pick up pattern specifically designed to pick up sound from a relatively wide area (the microphones used overhead to mic a choir for example) but the Shure SM58 is not one of them. The SM58 is designed to be a vocal mic for a single singer and the pick up pattern is configured to minimize the response to off axis sounds so that it captures mostly just the singer's voice. It is also designed to be used in fairly close proximity and you will see tons of videos of performers "eating" there SM-58. That's not a knock on the SM-58, they are great mics for their intended use and I own several of them.
So if you have a desire to use a single mic to simultaneously record both vocals and acoustic guitar, ideally you should use a mic that has a wider pick up pattern or higher sensitivity rating - or both.
As an example, if you have a camcorder or your phone can record video and audio, you will notice that you can capture pretty much the same balance between guitar and vocal as an audience would hear in a strictly acoustic, un-amplified performance. That's because the microphone on your video camera or phone uses more of an omni-directional pic up pattern.
Even with the correct microphone for your application, the ideal placement will vary (as you suggest in your question) depending on how loud the singing is relative to the guitar. Since you are trying to capture both vocal and guitar, with a pleasing balance between the two, I would start a few feet away from the performer and about half way between the sound hole and singers mouth - There are too many variables to say that closer to one or the other would be a better starting point.
Since you are asking the microphone to pick up two sources, you won't be placing it in close proximity to either source so adjust the input gain on your mic channel as high as you can go without distortion or feedback. Use headphones for monitoring to eliminate feedback from monitor speakers. With a more sensitive mic than the SM-58 you will not have to adjust the gain as much.
The halfway point is only a logical starting point from which you can begin making adjustments. The adjustments will be different with different mic pickup patterns - but if starting halfway between mouth and sound hole gives you too much guitar relative to vocal, there are actually several adjustments to try next.
First, try angling the mic towards the mouth and see how that affects the balance. Then try moving the mic higher and closer to the mouth but leaving it relatively parallel to to the floor (pointing straight ahead). Finally you could try even higher off the floor but angled more towards the guitar. All of these configurations will give you a different result and it will depend on the particular mic used among other things.
Conversely if starting in the middle gives you too much vocal and not enough guitar, try moving the mic lower while still pointing straight ahead and then tilting up or down from various positions.
It's a trial an error process but eventually you will find a sweet spot that gives you as balanced of a mix as can be achieved by using whatever single microphone you are working with. Monitoring the recording output through headphones as you experiment with various positioning strategies will save time.
It would also be extremely helpful to have a second person moving the microphone around while monitoring with headphones while the performer sings and plays at the same volume that will be used during recording. The second person would be able to hand hold the microphone and make adjustments on the fly and quickly hone in on the optimum height and angle and distance to achive the desired end result. Then it's simply a matter of adjusting your mic stand to put the mic in the position found to be optimal.
The first thing to realise is that with an SM58 you're never going to get a great result. It could be good enough to post up on Youtube and not be totally embarrassing, but that's about it.
As has previously been said, if you only have one mic to do both jobs then you need it to be a fair distance away, and an SM58 isn't designed to work like that. However - if you unscrew the mesh basket then it will do a lot better job. The mesh basket isn't just to protect the mic element, it's also to change the way the mic behaves with close range (proximity effect) and rejection of off-axis sound (to stop feedback on stage).
The downside of this of course is that the mic element is completely unprotected. If you sing straight into it then you'll get "pops", and if you hit it then you may damage it. So be careful.
Again as has previously been said, a second person wearing headphones who can move the mic until it sounds good will be a great help.