Nailed it!
Here's what I did. I started by making my paper just a smidge larger than the actual size. After sewing the PP section, I trimmed ONLY the edge with the point that would meet with the checkerboard. I left the paper intact and sewed the PP piece to the checkerboard piece. I then measured that finished piece and trimmed the whole block to 6.5" square. Then I removed the paper. The rest of the block I pieced with a scant 1/4".
The size is perfect, the points in the middle are close enough that I wouldn't bother to correct them.
Thanks all ... you helped me work this through in my brain. I'm now convinced that when you marry a PP block to block that has been sewn with scant seams, you could have a problem.