I haven't bought a gallon of washer fluid in forever. I used to swear by the yellow and orange Prestone blends for ice and bugs. But now, I just mix a little concentrated Windex in with plain tap water. Works great for me, doesn't seem to reduce the life of wiper blades, and dries streak-free. Works pretty good on smaller bugs, not so much on junebugs and the like, but multiple sprays and swipes seem to take care of even those. I don't even go by a formula. If the bottle is empty I pour in a couple swallows of the Windex concentrate, fill to 90% full with the garden hose, then shake the car from side to side to simulate mixing for my neurological issues. Never a problem.