I used to buy mine already mixed. When I began to mix my own I asked the local road superviser what ratio he used. If my memory is correct, he told me 7 to 1 when they stored it outside. That is richer than those who use 10% which would be 9 to 1 if I'm not too brain dead to think.
Since then I have bought from someone who mixed 12 to one.
I have tried a lot of different ratios and I agree with atgreene that dampness is the biggest issue. Also, the outside of the pile freezes first, like a shell. An advantage to less salt when doing gravel roads is that during warm spells it doesn't sink out of sight. So lately I have mixed a lighter ratio in the pile, then I throw pure salt over the exterior of the pile (to keep the "shell" from freezing). I cover the pile.
When I dig out a section, I throw salt over the exterior of the disturbed area. If the temperatures drop way down I throw extra on the area I'm digging and even into the sander. I also have a cover on my sander. I started with the cover because the little blue DOT trucks (or whoever they are that bust you) would hide on the back road I had to use. If they could bust me it would justify their existence goofing off. But I found that the cover kept my load dry and I could leave it in overnight. Check the spill on the outside, if it isn't frozen you are all set. If the spillage begins freezing check your load and get it off.