Generally anything below 4.0 is considered Hypo territory; I can go down to 3.5 with only a hint of shakiness. As long as you are above 5'ish then your blood and energy would be considered normal. The DVLA asks for a minimum of 5 for driving for example.
You are very lucky indeed my fasting including medication is double that, on Monday i had to Starve for over 24 Hour due to having a Colonoscopy on Tuesday and my levels 27 hours later was 8.2