320 feet is pretty high up in a hotel. What was the floor (floor 13 is non-existent in any hotel)? What is the distance from the base of the hotel to tje point of impact?
Anyone knowing about riflery will tell you that major adjustments of the sights must be made to account for the ballistics of shooting either uphill or downhill, in which in either case the on-the-level settings will cause the bullet to fall lower than the point that the sight picture indicates; and the farther away the more the deviation. Also, for a .223 caliber that might be considerable as compared to a .308 which reaches further out.
Doubtless in this case at the distance, inclination, and wind speed prevailing, in a milling crowd, in a rapid-fire scenario, an attempt to aim fire at any specific individual(s) would be quite unlikely.
The best one could do as a target in the crowd mass would be to move as rapidly as possible, laterally and in an erratic fashion, away from the center of the impact area into as area with as much peripheral cover available, solid or visual.
The worst strategy would be to remain hunkered down partially or completely exposed and immobile if one did not know where the shot were coming from, if the shooter was really sniping rather than just randomly sending plunging fire into the highest-density portion of the crowd.
Any infantryman knows that his unit needs to disperse whether under artillery or MG searching fire; and with personnel moving from cover to cover if it is a sniper seeking individual targets.
In this case, probably the shooter had observed and made allowances for the behavior of crowds, the location, and the density of grouping to know how to pre-plan his attack tactics with effective precision, eh?
Well it is evident much thought and planning went into the shooting and that there was plenty of premeditation