I would not like to address the other question posed by Ball and Belin.
What time did Williams leave the 6th floor?
No witness claimed to see Williams ascend to the 6th floor, other than the untruthful statements of Norman and Jarman, so that we have no way of determining his arrival on the 6th floor. His departure time is more important however as we have seen the discovery of his lunch in the SN places him in the supposed position of the assassin.
We do have some "time-point" events that can be used as markers however as well as rough time estimates. The first such "time-points" is the "elevator race" that occurred when the floor-laying crew left the 6th floor to break for lunch. When all participants statements are considered it is generally accepted that this occurred about 11.50am, 40 minutes before the assassination.
Here are the various statements of Williams as to when the elevator-race took place, when he arrived on the 6th floor, when he left to go to the 5th floor and why he might have departed.
11/22/63 - DPD
Elevator race ?about 10 minutes to 12?
?just after we got to 5th floor we saw the President?
no mention of trip to 6th floor
11/23/63 - FBI
Elevator race about 11.30am
6th floor at noon
No indication anyone on the 5th floor before leaving
Joined Jarman and Norman on 5th floor at 12.03 via the stairs
12/2/63 - SS
Elevator race ?shortly before noon?
Ate lunch on 6th floor for ?just a few minutes?
No indication anyone on the 5th floor before leaving
Joined Jarman and Norman ?prior to 12.15.pm?
1/14/64 ? FBI
Elevator race 11.40pm
Heard Jarman and Norman on the 5th floor
Joined Jarman and Norman about 12.05pm via the west elevator
3/19/64 ? FBI
Elevator race 11.40am
No mention of lunch or joining Jarman and Norman
3/24/64 ? WC
Elevator race 11.50am
Ate lunch ?about noon?
?Well, I thought I heard somebody walking, the windows moving or something. I said maybe someone is down there, I said to myself. And I just went on down.?
Joined Jarman and Norman after 5-12 minutes via east elevator
5/26/64 ? FBI
Elevator race between 11.30 and noon
Left 6th floor between noon and 12.15pm because he ?heard voices below?