Hey Ralph! Good catch: I never realized the cursor position isn't updated in the rulers when performing a drag…
Unfortunately there is no option to create a path by entering dimensions / distances, and the mentioned issue effects also resizing or dragging a path - which prevents some bypasses. Here's the best interim solution I came across: Create the second box with a starting offset of 4 mm, dragging its end point over the end point of the first rect. Now center the new subpath in the first box by eye … High zoom levels are improving a correct alignment, nevertheless it won't be perfect, sorry.
I'll put proper ruler updates on my issue tracker - hopefully there's a fix.
Edit: It looks like I was already able to correct this issue. Of course some more testing is needed, but expect a fix with StitchBuddy's next release. I will move this discussion into the "Bug Reports" section - hopefully you don't mind…