My hunch is he entered the area via the trail from the cemetery. Walked to the CS. Perhaps dropped a backpack or something similar.
Scanned the bridge. Saw his potential victims. Walked the creek side to the NW approach to the bridge, Looked back down the main trail from the parking area, where the girls were dropped off. Started across the bridge. Looked behind him maybe once, or twice, before he abducted the girls.
Just my dumb theory, and I came up with it shortly after the murders.
The above illustrates what I believe are the general distances of the abduction and murder, minus the trail to the bridge, and the distance from the CS to the cemetery. Just the abduction and murder. It's such a tight area, that I'd be surprised that once the abduction happened, from that beginning of the 4th leg of my stupid diamond, to their deaths, was under 20 minutes.
My time of death (TOD) is roughly 2:45 PM, and that might be pushing it.
JMO