Is there a formula for antifreeze

Pre Mix has always been the norm around here in Texas butttttt its getting colder and colder each year, got down to 18 ish last year and Im a member of the better safe than sorry crowd so I shoot for about a 60/40 mix. I fill with the 50/50 pre mix then top off with straight. My DD stays in the garage while at home and 8-9 hours on the parking lot at work, usually facing west so that nasty north wind dont hit it in the front.