Employ the Pivot Marking Technique

What happens here

To determine the precise cut line for a snug fit, position the capstone with its back edge flush against the retaining wall or the previously installed capstone. This flush point will serve as the pivot. Place your knuckle, or a small object of consistent width, underneath the outer corner of the capstone. While maintaining the flush back edge, pivot the capstone until your knuckle touches the retaining wall. The line created by this pivoted position indicates the exact amount of material that needs to be removed from the capstone to fit snugly.

Goal: Accurately determine the cut line for the capstone