I've done 90 degree corners for years on my binding. I willingly agreed to bind my daughter's quilt that had hexagon shapes on the edge of the quilt knowing it would be challenging. Not so much difficult as it just tedious. Angela counted the miters 2 nights ago; 256 outer points on her king size quilt, she did not count inside corners but I'm assuming the number is about the same. It takes 20 minutes to do the machine work on 7 miters.... That's about 12.5 hours of machine stitching.I'm spacing it out over 3 days. We moved the sewing machine to the dining room table so I wouldn't be hidden away in the sewing room. And I have the whole table to hold the weight of the quilt...very important, since it is so heavy.
I watched a few tutes about mitering and this one is good about the inside corner. Then this one about the outside corner.