This is a hard question. I think there are a lot of definitions, which makes it hard to pin down an answer.
The Davidson Institute uses a cutoff of 145 for services to what it calls profoundly gifted youth. So my impression is that 145 is PG around here.
I think of gifted as starting around 2 standard deviations above the mean. This is the top 2% and is 130 on 15-point standard deviation test. I think of HG as past the midway point between 130 and 145 (138 corresponds to the 99.4th %ile or so), but that's probably merely my own rambling definition. But it works for me, and if it works for you too, great! Then there will be two of us using the same terms.
Others use different numbers, such as 120 (~90th percentile) as being moderately gifted or mildy gifted (the label depends on the person defining it).
I've also seen terms such as "exceptionally gifted" but I have no idea how "exceptionally" is different from "profoundly." I can see adding a 160+ category called "really profoundly gifted" or something like that. Again, my terminology.
And then it's possible to muddy the waters even more by adding in creative abilities and what I call "thoughtfulness," which is a tendency to try to see as many different aspects of a question as possible.
