I also use a long 1/4" bit as a pilot. I use a hole saw on the soffit for a tight fit then I use a hole saw and a sawzall on the roof. The flashing covers the hole and making the top hole bigger allows you to move the mast around to get it plumb. There is a company that make a hole saw arbor that accepts 3/8 all-thread for extensions up to 10 feet. I have seen ads in the back of magazines like Electrical Contractor or EC&M.
