How much isopropyl alcohol do you add to your wash solution in the winter time?
Use methanol, not isopropyl alcohol. And there is no formula. There are too many factors such as wind chill, sunlight etc.
I use a bottle system so I don’t premix an entire bucket. However, if you do use a bucket, you might want to go easy on your mix, and carry a squeeze bottle with pure methanol that allows you to strengthen your mix on the fly.
Conditions can change from one job to the next, or from the sun to the shade.