Fantastic article and so true! I've seen this time and again with people who sight in just fine for 100 yards, but then struggle making their scope adjustments out to 200, 400, 600 yards and beyond. They will think they are moving their reticle 6 clicks up, but then it ends up going high right or high left. instead. Then they try going back left or right, and before you know it, they are all messed up. Then when they try to come back to 100 yard zero from the height adjustment, they are again all out of whack.
Here is my favorite scope leveling tool to use when installing the scope. It's cheap, quick, easy and seems to work very effectively. The Segway scope leveling tool.
Image courtesy Brownells
https://www.midwayusa.com/product/100910230/straight-shot-segway-reticle-leveler-mark-iii
Enjoy!
@ksteem