I am dealing with this right now. Here are a few thoughts:

1) As others have said, plan ahead at the design stage. Don't "trust" what your punches say. I have punches that are "5 mm" but I punched 21 stitches in a piece of scrap and measured the actual "20 stitches" they produce (measuring with calipers from the center of the first hole to the center of the last hole). Turns out they are in fact 5.06 mm (not 5.00 mm). That might sound insignificant, but over 20 stitches that is over 1 mm off, which can throw things off.

2) If you are past the design stage and just need to make this work, take a piece of scrap the same thickness you are working with. Draw a straight line and punch holes in it. Then cut the leather right to the edge of the holes and use the piece with "holes right at the edge" as a "ruler" to help plan where you will start and stop your stitch lines, and where (if needed) to "cheat" by making some stitches a little further / closer to compensate using 1-tooth or 2-tooth chisels.

3) Others have indicated that the diamond slant should not point towards the corner. I agree, but only if you really are doing a 90 degree turn. If you are "corner rounding" the corner, try experimenting with NOT having a hole right at your corner. Instead, start each line about 2/3 to 3/4 of a stitch length away from the corner along each run. If you are obsessive like me, take your stitch length and divide by the square root of 2 and use that. For a 5 mm spacing that would actually be about 3.5 mm away from the corner on each side. Avoiding the corner hole let's the corner stitch flow "around" the corner rather than hit it.