Because I used rocks inside the holes to stabilize the posts before the pour. Wetter concrete assured me I had complete concrete saturation around and under those rocks. No cavitation due to too dry a mix for that situation. I used the inverted mushroom hole type. No gravel on bottom.
Stabilized bottom with suitably sized rocks, then top allowing at least 2" above that topmost rock to concrete surface. Used level for plumb both ways.
After the pour, I re-checked plumb and made minor adjustments if needed.
I did the tensioning posts similarly, except, I used a 3/4" offset stringline for alignment with corners bottom and top. Stringline used for rough, removed, then installed again after pour for final alignment. Of course, the 90 degree opposite direction plumb was with a level.
A few neighbors in the area said to go with 5000 psi bagged concrete instead of the 3000 psi version. They said they had no cracking problems with it over time. Makes sense as sometimes 2 feet or more of the limestone rocks will displace when using a rock pick or auger at the surface. Leaving bigger area (hole) at surface to pour. Can't be helped. So, I used that instead. Dave