I guess my question is there a safe fallback positioning method on 12" guitar speaker? or is it always a case of rolling the dice? I guess this is another case for going direct with a Helix or Kemper or Axe Fx etc...
That's an "it depends" question, for sure. If you're working with a band and you'll be miking them up on stage on a regular basis, then, sure, experiment with mic position. You can do this at home or wherever you rehearse. Or if it's a big show, there will almost certainly be a sound check and that will give you a little time to experiment. Between a Shure SM57 and a Sennheiser MD-421, it will take just a few minutes of listening and moving the mic around a bit to get a decent sound.
But if it's a one-shot deal where a band comes on stage that you've never heard before, they have an amplifier you've never seen before, and they'll be off stage in an hour never to be seen again, then put an SM57 about 8 inches out and a few inches off center and go. You should be able to work with the console EQ to get it to work. It might not be like the perfect setup you've spent a couple of hours with, but when you don't have the time to fuss, you have to use what's most likely to work.
I do live sound at folk festivals, which, for the past 30 years or so, nearly always involve some bands with electric guitars. Frequently I don't know the band, may not know the music they play, or what they're plugging into their amplifiers. A Fender Strat, a Gibson L5, and a bouzouki all get plugged into the same Fender Twin because that's what the festival rented, and they all need a different kind of sound coming out (the bouzouki players expect it to sound like an ice pick in your ear). But I find that the old reliable SM57 always gives me something to work with. If you can't get a workable sound with that, it's because the player isn't getting the right sound out of his amplifier, and when you're on the spot, that's not a problem you can deal with.
Another thing, particularly with small venues, is that the amplifier is often loud enough on stage so that you don't need to send it through the PA at all, but if you put a mic on it and look like you know what you're doing, the player will think you're taking care of things. And if the player points to the amp and says "put the mic here," do it.