The white circle on Knoxville Road below (as the crow flies) is exactly 0.8 miles north from the vehicle of Bryan Hartnell and exactly 2 miles north of 5100 Knoxville Road (A&W Root Beer stand). This is extremely important because the police report stated that the three women who spotted a suspicious man observing them from the hillside "parked their vehicle at a location two miles north of the A & W Root Beer stand on Knoxville Road". This was followed by a second suspicious sighting of a man by Dr Rayfield & Son just over 2 hours later. They described a "white male adult subject walking in the area, about 5'10", heavy build, wearing dark trousers, a dark shirt with red in it, long sleeves". This sighting likely after the attack on the young couple and described in the police report as a "location approximately 8/10ths of a mile from the scene of the victim's vehicle". Two sightings in the identical area, alongside Smittle Creek Trailhead.
If the Zodiac Killer had parked at the Smittle Creek Day Use Area, left it there, and trawled the hillsides of Lake Berryessa traveling south, while looking for potential victims, then on his return from the attack on Bryan Hartnell and Cecelia Shepard he would have to pass by Dr Rayfield & Son at Smittle Creek Trailhead to reach his vehicle. That is why they spotted a suspicious man traveling across the upper reaches of the hillside sometime after 6:30 pm, wearing a shirt with red in it. The Zodiac Killer traveled a further 600 meters to the Smittle Creek Day Use Area, got in his vehicle and drove 1.83 miles south along Knoxville Road past the Park Headquarters. When he reached the payphone at Napa he mistakenly described this distance of 1.83 miles as "two miles north of Park Headquarters", accidentally giving away the location of where he parked his vehicle on September 27th 1969. Or maybe he was just terrible at distances - but great at cryptograms, codes, maps and radians. We all have to be bad at something.