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I'm with Pat here. I think that the formula is a good place for people to start, but then they have to evaluate what their weather is like, what their maintenance practices are like, and how they want to keep their chickens. I don't know how cold it gets in Vermont, but I'm guessing that you get quite a bit of snow. My chickens HATE snow, and we have frozen ground for about 5 months of the year. And when it gets really cold, they could be locked inside for days at a time. Last winter, when we had daytime highs that didn't get to 0F, I was VERY VERY glad to have 12 sq/ft per chicken INSIDE. Like you, my coop is 6x8. But I only have 4 birds. And personally, I couldn't imagine putting more in there. But that's me.
Look at it this way, with 22 birds in 48 sq/ft of space, you probably won't need to heat your coop. But you'd better make darned sure that you have really excellent ventilation or you will very likely have some frostbite problems on your hands.
Since you can't change the coop size, don't want to downsize your flock, and they are free rangers anyways, Pat's suggestion for setting up the warmest, driest and most snow-free outdoor zone possible is probably in your best interests. I'd consider roofing a run, making hay-bale walls or wrapping it with plastic to keep the snow and wind out, thus extending the size of your coop. Keep the inside for mostly sleeping or warming up.