You butt it up against part #88. Part 88 is what is actually made flush with the bartender side of the framing. Part 88 is 10″ long. The Base layer is 18″ wide.
The resulting base layer front overhang is about 3.5″ (3 and 13/23″)
Once the arm rail is installed that over hang will be increased a bit by the overhanging portion of the arm rail molding.
The confusion arises because part 73 is actually made flush to the back of the bar. You should start working from left to right and it all falls into place. Starting from the right side not so much. I do not see anything in the plans that says to make it flush to the back of the bar top support.
See diagram below…