I don't know if we have info on a normal range, but from
Ketones, Diabetic Ketoacidosis (DKA), and Blood Ketone Meters we have:
"
When do ketones show up on a blood ketone meter such as the Nova Max Plus or Precision Xtra Blood & Ketone meters?
The latest published research/studies in cats indicate ketones will begin to show up at readings of 2.4 and 2.55 on a blood ketone meter:"
That post was from quite a while ago now, but I think those numbers are still a pretty good landmark. I don't think it matters whether a cat is diabetic or now, ketones above that ~2.5 level are cause for concern, but lower numbers are probably fine (but should be watched very closely if there are any associated symptoms).