Scope bracketing and true power level

I have a AirMax30SF and compete in HFT with a AA Prosport. 

First is it 10 or 12 power that it's, I guess the word is, "calibrated" for or for it's reticle to be 3.6" from center mildot to center of next mildot at 100yds?

Secondly, after finding if it's either 10 or 12 power to be true at 100yds, for argument sake, say it's 10x. Well at 25yds, it should measure .90"? And what is the closest ruler marking for .90"? 7/8"? Or should I use 23mm? 

Now for bracketing. If a brick is 7.2 inches and it spans 2mil at 100yds, does that mean 4mil at 50yds and 8mil at 25yds? At the 10 or 12 power the Airmax is true at. 

If so to all of the above,then what would it be on 16 power? Reason I'm asking is because at the moment, since I moved to Delaware,I only have access to 25yds. I will hopefully be able to continue my range card for 30yds and beyond in the near future but just for bracketing sake.

Hopefully I am making sense with all this. Looking forward to seeing your feedback.

Cross posted