A chair comes in handy for getting that amp off the ground
My thoughts on mic'ing a guitar amp...
I think a good starting point is between the cone and the edge of the speaker. For an SM57, I suggest starting by pointing it slightly toward the edge of the speaker. As far as dynamic mics go, I like the SM57 and the Audix i5. The i5 is a bit brighter. I pretty much like to shove them up against the grille.
As far as ribbon mics, I like the AEA R92 and the sE R1 Ribbon. Follow the manufacturer's recs on distance. In essence, you're not going to be shoving one of these up against the grille. Usually, for me, distance varies from six inches to one foot. I go with the same method as with the SM57. I point the mic between the cone and the edge of the speaker.
I prefer getting the amp cabinet off the ground.
I prefer the volume be adjusted just so the amp sounds good in your room and not any louder at all. Don't try to over power the room.
I do not like putting big amps in small rooms. I've never had it work that I can remember. I've been to some studios where they like to put big amps in closets. I got no idea what they are trying to do as it always sounds like crap.
You can use a flashlight on some amps to see through the grille cloth. On some it does you no good and you'll have to guess. On other amps, the grille cloth is transparent enough that you'll not need the flashlight to make a decent placement.
Everyone acts like guitar amp sounds are fairly easy. I've found it among the hardest things to get right.
I recommend using a pair of Extreme Isolation Headphones or other quality isolating headphones to fine tune your mic placements.