Here is a chart for Propylene Glycol.. AKA RV anti-freeze. Note as it usually comes from the bottle it is rated to -50 degrees F. So as you can see on the chart this is about a 55 percent solution. So as you can see if you cut this 1/2 and 1/2 with water, you would be at about 27 percent with a freeze point at maybe +12 deg F. Is that enough for your area? On my last tractor I used it straight from the bottle, with 1 gallon in 24 extra pumped in for a total of 25 gallons each tire. In my Kioti I have Methanol solution. as we made a new purchase deal with the dealer. Adding a small amount of isopropanol is not going to lower the freezing point a lot. Ordinary isopropanol is usually 70% straight from the bottle.. Although there are higher concentration blends. You just need to find a chart for it like I did with the RV anti-freeze. Yes a lot of the "summer blend" of windshield washer fluid freezes at +32 deg F.. same as water. Winter blends, some is 0 deg. and some is -20. I have never seen any around here lower than -20 F.
James K0UA