When I trim a foot ring, I also like to measure the pot thickness with a needle tool first. I find this is a big time saver. I measure in three places: the outer edge of the foot ring, the inner edge of the foot ring, and the middle of the floor. Once I know these measurements, then I trim straight to the right answer. No hesitating or guessing, and no trimming through the pot.
A lot of the pots I make in multiples do not have foot rings, for those pots I skip the needle tool. Instead I eyeball the shape of the interior, then trim the exterior to match the shape of the interior. (my point is ... foot rings are not required, unless you want them.)
Anyhow, whether I'm measuring with a needle tool or not, I always gauge the "dentability," i.e. when the wall is the correct thickness, it can be dented with a press of your finger. I taught my students "when it's right you should be able to do this" then I'd make a big dent in the pot with my thumb. When everyone stopped gasping, I'd invite them all to dent my pot, so they know how it should feel. Then I explain how to check for dentability without making a visible dent ... press until you feel the wall begin to give, then stop, it will bounce back. I will also make sure to say "if you press and it still feels like a solid object, keep trimming."
This is similar to what others are saying above, teach potters that pots can be ruined and life goes on. It just means you get to make another one.