When geometry matters

I've spent parts of  the last few days trying to figure out exactly how to lay out my Diggity cards.  I've got an artist who's hopefully going to work on the game, so I need to give him some good guidelines, and the layout has been bugging me a bit during playtesting.

The card layout matters, because the cards act as tiles - players need to line them up to play them properly, and they're not allowed to overlap the cards - see the picture included in this post to see how it plays out.  There's one connection spot on the short side of the card, and there can be two on the long side.  In the initial design, I put the one connection in the middle of the short side (that was obvious).  For the two connections on the long side, I put them 1/3 of the way in from each side, which also seemed obvious.

But it turns out that position leads to occasional confusion.  One of the rules is that the cards aren't allowed to overlap when played, and given the 2.5"x3.5" size of the cards,  the one-third offset actually creates somewhat frequent questions about whether cards are overlapping or not.  The 1/3 offset looks nice; it's a natural ratio.  But shifting the connection points just a hair towards the outside seems to help a lot. It's a tiny shift - about 0.07 inches - but it makes a big difference in how the cards connect, and it doesn't change the look too much.

This seems crazily specific, but it's something that playtesting has really helped with.  Not blind playtesting, which is great for rules clarity but might miss this kind of thing.  What I needed here was to play my own game a bunch with other people and watch how they play and interact with the components.  Can't take any shortcuts, it seems.

UPDATE: It appears I'm not alone in sweating the small stuff