Well if you say one AD against one Balloon your math is correct. But in a maxed out base. The balloon might be targeted by several anti-air defenses (xbow air/ground, AT,IT and WT). So you should make several combinations of a balloon being hit by one the above and AD at the same time. Then you will have different results. I won't upgrade them at the beginning as they will be right before the cannons after every thing else.

