I couldn't find any information about the ideal temperature for freezing the nafta with goodies. So I did a little experiment.

My hypothesis:
When the the temperature of the solvent is lower, then the solubility gets less. So the result of a lower temperature will be more crystals.

Setup:
I still had some nafta left from my last extraction. When I freezed it at -3C/26F and collected the crystals. I took this same jar with 'leftover' and put it in the freezer at -12C/10F. After 12 hours I took it out.

Results:
There were no crystals. This is evidence against my hypothesis. So it seems the freezing can be done at any temperature below zero Celsius. Ofcourse to verify this you can split one pull and freeze it at different temperatures. This setup could also be used to prove if you can speed up the process by lowering the temperature.