You will be filing the groove into the forms by pushing the filing tool back and forth between the form halves until the sole of the tool rests level on the surfaces of the bars and no more material can be removed. When this occurs you will need to close the width between the bars at each station by .010 inches so that you can remove more material. When the tool again ceases to remove material you will repeat this process of closing the distance at each station by another .010 inches. You will be repeating this process quite a few times (particularly when filing the deeper groove on the butt side of the forms). As you approach your target dimension you may want to go down to only .005 (or even less) in order to avoid taking off too much material and ending up with grooves that are too deep to make anything besides salmon or bait casting rods. Note that in this image strips of wood have been clamped to the table to serve as runners for the filing tool to slide onto when it passes the end of the forms. This better enables you to do a consistent job of filing the groove accurately, and may also save you some badly bruised knuckles