I actually don't believe that all-day fasts are useful. After a certain point, it is just an unnatural condition and my body reacts badly. I do mini-fasts. And even with mini-fasts, I can't really do it all, for instance mornings. If I fast in the morning, my blood sugars go south, they just rise and rise, hardly my natural pattern.
The point of your basal is to keep your fasting blood sugars between meals right on target, not to provide the level of insulin you would need if you fast all day.
And if you are going to fast, you need to really fast. Just small amounts of water. Eating anything can disturb your blood sugar. Certainly no protein, I have to bolus for protein. I count 50% of the grams of protein as carbs. And if you eat a volume of anything, even water and ice, it can induce an effect on your blood sugar (the "Chinese Restaurant Effect").