I'm going to crack on with this supposed debate with Jack and assume he's going to join in at some point.
Firstly I'm going to assume that Jack does, indeed, believe that Arnold Rowland invented the man on the 6th floor. I've asked him for a Yes/No on this issue at least six times and he has been typically evasive about this most basic question. I don't know why and I'm not sure I care but let's assume his evasiveness is to hide the fact he believes something so fundamentally ridiculous as the Miracle on Elm Street.
Secondly, I'm going to choose the most coherent thing I can find that he has posted on this subject which, with Jack, is no easy task. I'm going to pick what I believe is a valid point that needs explanation. Unfortunately most of Jack's 'evidence' is just his wobbly opinion of things but here goes.
This is from REPLY#2021, "The First Shot" thread.
Jack doesn't seem to believe that Rowland could have seen the man with the rifle through the open set of windows in the way he describes in his testimony (I think):
The window is 29 inches tall and it starts 14 inches off the floor. He doesn’t know that because they finally ask him if he had ever been inside the TSBD. No
But his description of the person in no way reflects the correct window dimensions.Mr. SPECTER - And how much, if any, of his body was obscured by the window frame from that point down to the floor?
Mr. ROWLAND - From where I was standing I could see from his head to about 6 inches below his waist, below his belt.
Mr. SPECTER - Could you see as far as his knees?
Mr. ROWLAND - No.
Mr. SPECTER - And what is your best recollection as to how close to the window he was standing?
Mr. ROWLAND - He wasn't next to the window, but he wasn't very far back. I would say 3 to 5 feet back from the window.Mr. SPECTER - How much of the rifle was separated from your line of vision by the window?
Mr. ROWLAND - The entire rifle was in my view.But he is not done yet. He is unstoppable with his description of what is impossible to see through a 29 inch window. Specter realizes he is making it all up as he talks.Mr. SPECTER - And how much of his body, if any, was in the open view where there was no window between your eyes and the object of his body?
Mr. ROWLAND - Approximately two-thirds of his body just below his waist.Mr. SPECTER - Up to what point?
Mr. ROWLAND - Mid point between the waist and the knees, this is again in my proportion to his height that I make that judgment.
Mr. SPECTER - So from the waist, some point between his knees and his waist, you started to see hi clear in the window?
Mr. ROWLAND - Yes.
Mr. SPECTER - And from that point how far up his body were you able to see without any obstruction of a window between you and him?
Mr. ROWLAND - To the top of his head. There was some space on top of that where I could see the wall behind him.
Mr. SPECTER - What is your best estimate of the space between the top of his head and the open window at the perspective you were observing?
Mr. ROWLAND - Two and a half, three feet, something on that--that is something very hard to ascertain. That would just be an estimation on my part.
Clearly Jack does not believe Rowland could see as much of the man with the rifle as he said he could, because the gap of 29 inches was not big enough. He describes it as "impossible". I hope this is a fair summary of Jack's position and I'm sure he'll correct me if it's not.
As far as I'm concerned, the only way to accurately test this claim is through computer graphics. It would require an accurate model of Dealey Plaza and the TSBD building, with the ability to have a POV from Rowland's known position.
And this is exactly what Charles Collins did in "The Floor-Laying Crew" thread (Reply#138)
Okay, here's what ~4.5' looks like in my 3D model. BRW sitting on the cart is the camera position and is represented by the virtual visitor icon. I don't think it is quite that far, but giving you the benefit of the doubt, the stack of boxes is approximately 4.5' from the inside of the south wall. I have made one box invisible so that part of the rifleman character can be seen behind the boxes from BRW's position. The rifleman character is about 5' from the inside of the south wall, within the revised range that Rowland estimated.

Here is the view from Arnold Rowland's position. It fits reasonably well with his description.

Charles was actually trying to ascertain what BRW would have seen from his stated position on the trolley at the end of the third aisle while having his lunch. He established that boxes down by the east end of the 6th floor would have obscured the man with the rifle from the view of anyone sat on the trolley, if the man was stood about 5 feet from the southwest window.
He also showed what someone stood at Arnold Rowland's position would have seen through the fully open southwest window.
IT IS A PERFECT MATCH FOR WHAT ARNOLD ROWLAND DESCRIBED.
Could there be clearer confirmation of the accuracy of Rowland's testimony on this matter?