Hi Jose,
The middle position is always a good place to start, personally I use the mastbase a bit more forward on our boards. To give you an idea, I can see +/- 1cm of the mast track in front of my mastbase.
So maybe try the mastbase a touch more forward and see how it feels! In general it's always good to try all possible positions to get a feeling :-) Sometimes it doesn't work, sometimes you discover a magic setting!
Enjoy and if you struggle with the trim, just let us know the problem!