This was my first attempt. A week's worth of tatting time just to see that it looked like crap. Grrrr! The arch between the motives ruffled like mad and just wouldn't lie flat. The chains around the corners took away all of the square definition and made the corners round. Really ugly and no way to fix it except cut it off.
Once I cut the offending border off I had an arch section of the exact proportions that I could play around with. I didn't want to do anything elaborate on the corners because I wanted the focal point to be the arches, but I also didn't want them rounded.
Here's the final result and I'm quite pleased with how it turned out. The negative space created pattern within pattern so that the joined motives look more like a planned whole than just a collection of motives. Your eye is drawn to the X created between the motives and it takes a moment to see that it's just 4 motives with a border.
Sorry to burst the bubble of those who think that designers just "get it right" the first time. The truth is that sometimes it's just a lot of slow slogging doing it over and over again until you like the results.