Here is the 2nd floor lunch room, and you can see everything.
So, the distant door is the door with the window through which Baker was looking from the stairwell. There was one like it on the other side, out of view to us. And the door to the lunch room proper is front and center and sprung open, just as it was that day. Oswald came from the office side through the swinging door and went directly to the center doorway and to enter the lunch room. Baker saw him right when he was in that passageway room.
And when Baker got into the passageway room facing the lunch room, he saw Oswald walking, apparently at a normal pace, through the room. The angle of the above view does not show the Coke machine. This does:
There it is on the left. And I have to think that Baker saw Oswald before he got to it because he would have been practically out of the room, plus, it would not have taken Baker that long to get Oswald in view again.
Baker specified the distance: 20 feet. Of course, it was just an approximation.
So, where was Oswald going. What exactly was he going to do if Baker hadn't called him back?
Well, judging by the look of it, one could argue that he was going to get a Coke. Again, I don't think he passed the Coke machine yet. And after Truly and Baker left, he did get a Coke.
Beyond the Coke machine there was another door which led to what looks like another passageway to the Conference room. But, it would be arbitrary to say that he was going there. What he, in fact, did was turn around and approach Baker without saying anything, which I find VERY STRANGE. I should think that if a cop was pointing a gun at you that you would say something, maybe even, DON'T SHOOT. I HAVEN'T DONE ANYTHING. And I should think that one might instinctively put one's hands up to show that you're unarmed and not a threat. But, Oswald didn't do anything. He just walked up to him, saying nothing. It's strange.
And then, when Truly and Baker left, Oswald got his Coke and then reversed his steps and took the same route he took to get there, encountering Mrs. Reid in the office area.