I think you'll find the answer to your questions in this thread:viewtopic.php?f=46&t=16635
It's a common question, and there's lots of good explanation there. I'll leave you to your reading, but the short answer is...you cannot do cuts by temp. The temp of the vapor coming off solution in the pot is determined by alcohol content of that solution. Further, that changes throughout the run as the alc is depleted from the solution (ie., the temps continually rise). Because of that, you can have heads at 90%, or you can have heads at 65%...just depends on where you started.
I see "P" beat me to it