modelling particle curtain
 

Im modelling heat transfer of sand particles in ambient air in a rectangular box with E-E approach.At the top of the box there is slot in which particles falling down and the size of the slot is 10 mm,and the box size is 450*900*225mm.
The problem is in my geometry when I'm measuring everything to make sure that the slot is in the middle of the box,everything is symmetric at both side.But in my results when I put the line at the middle of the box,the middle line is not at the middle of the slot.I don't know if the slot is not in the middle of the box or the line.But the coordinates are ok for both.
This problem makes the particles curtain unsymmetric!
I was wondering if anybody could help me.