I have a P337G with boots. I I have been in ice too many times and Larry is right, avoid it. You were wise to turn around.
Now to answer your question. Procedure for my plane is not to use the boots until there is abut 3/4 inch of ice on the leading edge. The Skymaster will really carry a load but to tell you the truth, going into freezing rain is never on my agenda. Better wishing I was in the air when I'm on the ground then wishing I was on the ground when I'm in the air is my motto.
Jerry
ICE
