I've realised how misleading my post may have been when I said 3.9 seems to be a common low point. I should have stated very clearly "for non-diabetics!".
Like you I was concerned when I saw 3.9, so looked up what "normal" (non-diabetic people) get, and was reassured that it's common. A lot of people say that someone with T2 should aim to mimic the blood glucose patterns of a non-diabetic, so I reckon if you can find a way of life / way of eating which regularly gets your fasting numbers that low, then you are doing very well, keep it up!
As for "normal" for someone with T2, well there is a thread somewhere on this forum where people compare morning numbers, and 3.9 is far from normal there. Personally if I get less than 5 I feel I've done well. And I find it takes effort. I like to think that I could get a 3.9 again if I did exactly the same thing that achieved it last time, but I don't think that's a way of eating / way of life I'd be happy with - I don't think it would let my brain function well enough to do my job!