The way to Jim an even curve,is to screw up the Jimcrow,the same number of times,at each point,marking where the screw is positioned,after giving it a whack with the sledge at that point,it helps stopping the spring in the rail from pulling back,then moving the grip end ,to where the screw position was marked,and so on.
A good eye is needed,and regular gauging on the 2nd turn rail.