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Neil sent me this link to a cool trick for holding round-head screws while you cut the threads shorter. Cut a thin slit in a correspondingly-threaded nut, through one of the points. A rotary tool with an abrasive disk is probably a good tool for this. Then you can thread the screw in and grip the nut across the flats with pliers or a vise. Compression across the width of the slit will hold the screw firmly in place, and you can use the flat side of the nut to guide the saw.