I think that as you pry the shingle away from the billet you are also pulling away from the billet, and this must be why you get the taper. I don't know what you are making the froe out of, but I know it is a lot of work to beat one out at the forge. as you are drawing the steel to make the edge, the steel forms a crescent, with the cutting edge on the outside. A froe also does not need to be all that sharp either, as it is splitting, not cutting ou the piece.
Dave
