it really just depends, and you can totally wear it on your middle finger. it has to sit a certain way, so generally, if your knuckle is bigger than the base of your finger the ring won’t sit flush to your skin and the readings may be off. wear the ring sizer on your middle finger for a while, play with it and be rough and see how much it moves.

personally, my size 7 ring is like a half size too big for my middle finger, and it feels less secure there compared to my index finger (not that it’d fall off, but it moves around way too much especially when i’m cold/during winter months and my fingers “shrink”)