Don't know if you ever tried trimming with a jar lid before but it is a great way to hold the pot on the chuck while dispersing the pressure. And it is also a visual reference so you can either have a bunch of lids and just trim to the edge or a few and keep it a 1/4" away, etc. That way your feet are all the same if you want them that way.
Just center it an hold it with your finger and trim away. I sometimes add a drip of water to lubricate it.