I have long wondered the question you are asking. My goal is similar, I don’t want to increase carbs for bad foods, but purely to be able to enjoy some small amount of fruit and veggie without having to worry about weight gain. I so desire to have a bowl of fresh picked strawberries…unfortunately for me, I’m not one of the lucky ones but you may be. My husband can easily intake ~70g and still register blood ketones. From my perspective I wanted to ensure he was still using fat for energy due to the amount of fat he was eating in the day (wanted to ensure it was being burned up).
From what I’m gathering from your question, you want to know that borderline level so that you can eat more carbs but still be considered as in ketosis so that you are in fact burning fat for fuel and that you don’t get any of the nasty side effects like keto flu. I would think then the only way to find out is testing your ketone levels. Increase carbs by 5G each day until you find out that threshold bc that will be very individual.