https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6472268/
There is no specific ketone cutoff level specified, and of course the impact of X g of carbohydrate will vary per individual, during the day and between days.
The paper I cited are the end stage validation that a treatment works: using it on patient in their normal everyday life (instead of being in clinic/laboratory) - note that they see the doctor 3 times 10 minutes per year.
Of course no one knows how much carbohydrate they take but the guidelines (as presented in the paper) are clearly steering them to nutritional ketosis.
As or clinical studies there are many of them done in the past decades, for a ten years ago summary for example:
https://web.archive.org/web/20150702005325/http://www.omega3...
Dr Westman (the main author, Past-President and Master Fellow of the Obesity Medicine Association and Fellow of The Obesity Society, he also has a youtube channel) uses 20g/day carbohydrate guideline and does not recommand the use of a ketone meter for his patients.
A ketogenic diet involves severe carbohydrate restriction and even major protein restriction. The research you shared isn’t applicable.
This is a huge problem in online keto discourse: People share a lot of “research” that isn’t applicable, such as these low-carb papers or other research about treating epilepsy with keto diets.
You can’t just look for studies about lower carb with positive conclusions and assume that this implies the keto diet is good.