With the stud riveted and soldered to the lid, and the catch/spring installed in the end grain of the buttstock, I coat the stud with felt pen, slide it down into its mortise. the catch will score a line on the felt pen ande there will be a definite stop when the lid is bottomed. Re-open the lid and use the mark created by the catch itself to start your filing. Allen comes at it on an angle so that he can approach the line gently, and thereby, have no slack in the catch at all.