Experience. I’ve been using and experimenting with ATS for 20+ years. Learned a lot by my mistakes. Blending procedures play a lot of it. Little things like putting the least used product in tanks first and pumping UAN through that to correct level can mean the difference of it being ok or salting out. 
I figured 90-10 at 15 degree salt. 85-15 should be 7-10 degrees. 

It’s all about getting the N number lower. 

I’ve taken 80-20 Hydrite product made with Molten Sulfer to -10 in a freezer with no issue. Tessenderlo-Kerley product made it to 0 before that blend salted. 2 different processes to make same final analysis of 12-0-0-26. Hydrite salts a 25 degrees by itself and TKI salts at 40 degrees. Seems to play out to help the blends.