Z Seam Alignment Not Accurate

[MakMakMo 0]

I have a disign similar to a square.  I want to align the seam at an exact specific point.  I can set the seam to x 50, y55 and I can see the seam start and end on the corner.  But I want to move the seam slightly to the other side of the corner so I try to set x 51.  The seam now jumps about 5 cm the other direction.  I can even make adjustments to .001 and it will still jump almost 5cm the other direction.  What is the deal???  How can I get accurate full control of the alignment?  Also is there a way to put a point on the surface to see the exact coordinates I want to pin point rather than trial an error every fraction of a millimeter? 

[burtoogle 516]

The z-seam is only ever positioned at an existing vertex. Perhaps your model doesn't have a vertex at the position you wish the z-seam to be located?