It's math. If it's 6 quarts total, then you now have 50% = 3 quarts anti-freeze in there. If you want 40%, then you want 40% of 6, which is 2.4 quarts in there. So you want to remove .6 quarts of AF. So you'd remove 2 x .6 = 1.2 quarts of 50/50 mix, then refill with water. I wouldn't bother. I live in AZ, and that 50/50 vs 30/70 debate is not resolved. More water (as in 30/70 mix)means better heat transfer, but also means a lower boiling point. It's a tradeoff.