Caught me out last Sunday. I've just upgraded from same Sidewinder as you to a Big Nikko. Set it up at 18 Degress, got to Thorndell last sunday and there was frost on the ground :-( walked off with the lowest score to date. Like Rob says' its all about trying at different temps and seeing how it behaves.
As a side note, i did my zero, paralax and clicks on the sidewinder last year at about zero degress and it seemed to stay consistent right the way through winter. only saw a real shift as we got towards summer.