need pattern brush strokes to maintain the same spacing no matter the size of the shape

Hope someone can help me. First I am new to AI but not to PS so I have half a clue maybe.  So…. I do leather work and am starting to create my patterns in AI. No problem with the shapes and the like but when I go to add a stitching line, I created using the pattern brush and apply it to say a box and then to a different sized box that would be sewn together the stitching spacing doesn’t match up. So I guess what I am trying to say is I need my 2mm high 45 degree diagonal lines that are 3.38mm apart from each other (that’s the distance between the pricks on the pricking iron)  to stay that way no matter what the size or shape. Make sense?  I know it can be done I see it on other patterns. Any help wouyld be greatly appreciated. Thanks in advance. 

on attached picture to differnt sized squares and you can see how they start to offset on each other. thanks the problem.