Slush and solid is just a matter of time. The differentiation is thermal mass (how much heat the engine block retains.
You could make a practical assessment of how often and for how long is the temperature actually -15? Maybe not long or never.
Still, to me that means your as is a little light, as it still gets quite cold in IL. I'd just siphon a small amount of your current AF and replace it with neat AF and call it a day. You can do a calculation to determine how much you need, and you definitely don't want to get too much glycol in there, but you should add some.