I've had the same problem: "Lower" middle grade would be closer to easy chapter books, just plain "Middle grade" would be regular 50K MG stories, but most of my stories were in a sort of transitional period where a 12-14 character has to start thinking serious thoughts, but is too young to be YA--
YA stories tend to be pessimistic and put the character at odds with a complicated world, MG is more optimistic, and has the character trying to get ahead and accomplish something cool....Anything in between, I tend to term Upper Middle Grade.