I'm just getting into milling, bought a new 660 and the salesperson convinced me that square filed chain is the way to go. Having never even sharpened a chain with a round file, I previously bought a Granberg FILE-N-JOINT hoping this would save me. I sucked at square filing, but I found a way to use the Granberg Jig to get what I think is a very good square filed chain and I thought I would share my method. Looking at the picture, I file from the inside but look at the outside corner. For this picture I would support the tooth with my left thumb and draw the file with my right hand. The file sits diagonally in the holder and is held in place by sliding the ends together. The thumb screws to hold the round file are not used.