The timing of 3:00 pm is correct within just a few minutes, based on the doorway shadows in this Allen photograph compared with my 3D model. The simulation runs from 2:45 to 3:15.

Thanks James.... It certainly appears that the time that Montgomery and Johnson departed the TSBD with that huge paper sack was around 3:00pm.
I wonder if you've used the shadows to determine what time Detective Day left the TSBD with the carcano. I believe that Day departed at about 2:10 and returned to the TSBD about an hour later..... Or around 3:15..... If that is true, then Montgomery left with the paper sack before Day returned, and Day never saw this bag in the imaginary "Sniper's Nest"