A fly ball in this situation is judged by the plate umpire. The 3rd and 1st base bags are a good line up point, if there happens to be a fair pole on the outfield fences this really aids in lining up. Now when the ball is caught and it appears really close, its my judgement to call it fair or foul. With trickle grounders that pitchers, catchers or basemen are trying to hustle because they are trying to get the out thinking it will stay fair is doing such at their own risk. Lots of times the ones that obviously can’t be retrieved fast enough players let it roll until it comes to a complete stop. Some players will stop and retrieve a zig zag ball while in foul territory just to kill the play, same as the ones who grab it while in fair play but the batter runner was to fast. If they choose to grab it in fair territory, fair ball, if they grab it in foul territory foul ball.
I actually prefer to call it the fair line not foul line. If ball comes to complete stop on the line....fair ball run all you want.