Trouble trying to figure out ring size

Hi everyone! I feel like this may be a common thing but I am struggling. I received my ring sizer kit last Friday and still haven't decided which size feels the best. I'm between a size 11 and 12. The size 11 fits my left index finger and left middle finger. It does leave a bit of an indentation on my index finger after wearing it but it's not really bothersome. It's definitely not going anywhere once it's on my finger. With the size 12, it fits and feels like I'm not wearing it but it also feels a bit big. I can move it up and down with my thumb. I know that they say to size up if you're in between sizes but I'm wondering how much that will mess with the tracking if it's a bit loose.