Hi Paul,
My dad used to cut loads of glass as part of his job and if i recall, he used to make a single score line, then tap from the underside with the ball end of the cutter along the cut to deepen the score line before attempting to snap the piece.
He would usually put a thin piece of timber like a doorstop (about 6-10mm thick) just back from the cut to lift it up slightly before applying firm pressure to snap it.
I've only ever seen a few bust on him using this method.
Mind you, mirror glass might be a little different nowadays, I think most pre-cut ones have a safety film to stop them splintering into shards when they break, but I would assume standard mirror glass would be the same as ordinary float stuff, so it should cut in the same way.
Andy