The take off position isn't controlled in the mission file other than by placing yourself as one of the wingmen as Vidal described. You are correct however in that you'll find youself slightly behind the other aircraft when you do this. Also, there are some other benefits you lose when you switch yourself to the wingman.
I think you'll find that changing the units_across= information will have the greatest effect on the take off formation. On some of my WWI airbases I have the units across set to 8 since the runways are nothing more than large grass fields and this simulates a full flight taking off abreast. Still, as I recall, the planes stagger slightly so the lead plane is ahead of the number eight plane. I don't remember if they form up to the left or the right of the lead flight when on the line.
Of course a setting like this would be useless on a conventional runway as most of the planes would be off into the dirt and might run into revetments or hangers.
I don't know that anyone has looked into changing the configuration of the take off formation before. It might be hard wired into the code or maybe you'll be the first to find a solution the rest of us have overlooked.