I believe your right on the formula but I don't know beans about how you determine the critical loading of a particular plane. That T-6 is beautiful. One of my favorite planes but for some reason have never built a scale one. I blew up a set of house of Balsa 15 size plans to a 60 size once. It was a fantastic flying Sunday flyer. I'll have to build me a Ziroli one of these days. (or just buy one, that sounds easier).