I'm no statistician, but I've always been dubious about that calculation.
You are taking the mean of two means, which seems to be getting farther
and farther from actual data.

It also seems to punish some of the climates that are best suited for
natural ventilation, since having cool nights significantly drops the
acceptability limits. I assume they are pushing you to take advantage of
night pre-cooling and thermal mass, but I find it curious that there are
no allowable hours outside of the range. All of the pre-cooling in the
world isn't going to help you keep the temperatures down during that one
string of 90 degree days in the tmy2 file...

Nathan Miller