I could be wrong but I believe it is the amount of pwd sugar to fat (butter/shortening). I know you need lots more pwd sugar to fat but I'm not sure of the exact amount. When I make buttercream I use 3/4 cup butter and 1/4 cup shortening and 4 cups pwd sugar. I usually add a little more pwd sugar to make sure it crusts. Hopefully someone can give you a better answer! 
I don't know what the magic ratio is, but it is definitely the fat/PS ratio that makes it crusting or non-crusting.
I know the "typical" crusting BC I have seen has 1 C fat/1 lb PS.
What is your recipe?
And yes it is the ratio of fat to powder sugar. The more fat the less crusting, the more sugar the more crusting.
Could you please post the actually recipes in this thread or the link for it? I went to look for it in the files and can not find it.
Okay looking at the recipe and doubling it because I'm use to looking at the fat to sugar for 1 to 2 cups of fat.
So it would be 1 cup of butter to 5 1/3 cups sugar. The recipe should crust. I don't use butter, but I believe it does make a softer icing. And the amount of liquid seems like alot to me, but then if you add alot of cocoa powder to it I think it will be okay. I usually have to add more liquid than I normally would when adding chocolate.
Interesting...I thought whether it crusts or not has to do with whether the liquid gets mixed in with the sugar, as opposed to having the fat and sugar mixed thoroughly before the liquid is added. I thought the crust was actually dried sugar that was dissolved in whatever liquid is in the recipe.
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