So now that I have the reference post, I realized that my holes were slightly off. So today I expanded the overall hole to enable the next two posts to be placed square with the reference post. I also wasn't satisfied with the post hole for my reference post, so I added a few inches to the diameter of the other post holes.

Tomorrow I will try to place the posts and keep them as close to square as possible. My theory is that the distance between the two post corners should be close to the calculated diagonal distance from my plans. I got really annoyed with FreeCAD, and I was too tired to do trigonometry, so I spent about a minute sketching the post placement in Inkscape so I could measure the interior corner-to-corner diagonal distance.

Obviously this isn't going to be accurate enough to be exactly 74.95", but we'll call it 75" +/- 0.5" and hope for the best. Which probably means I'll be posting about I couldn't achieve that precision either without digging even more :-)