My tank is small enough that I use the 10% methanol summer blend in summer, and the 40% methanol -35C stuff starting in October when it comes out. By the time the windshield freezes (today), there is enough of the winter stuff to not cause a problem. But I think that I use more windshield wiper fluid than most. You can also buy some methanol at Home Depot to spike what you have. I will go top up my tank tonight with some methanol that I have.
I always buy the stuff rated to -30F for winter because it seems the -25F (typical winter WW Fluid) fluid will freeze on the windshield when the temps approach 0 deg. F.
Finally someone feels my pain! I've never understood that! How can -25F fluid FREEZE on the windshield at +25F???
The problem is that the alcohol is coming out of solution due to evaporation....then you're left with higher concentrations of water. One way to cure this would be to install some sort of check valve.
An easier option would be to keep a bottle of isopropyl alcohol handy and add some of it to the fill bottle every so often.
I know this is wierd, but I have an older bottle of ST blue outside and it froze at about +20f! I had poured some into my nephews car, so I pumped it out and refilled with new. I had a wierd feeling when I used it, not shure if it was car wash or what, but definately not because carwash is not blue. It froze solid. Funny story everyone hee should know, is how the Russian Mob smuggled dyed-blue vodka into somewhere, in washer fluid bottles. They could get the blue colour out later somehow. I still keep a bottle of "washer fluid" under the seat.