I assume you mean "mean", not "medium"?
However, calculations that take in just HP and armour aren't sufficient for determining strength. There's tons of other factors, including attack strength, attack speed, cost of the unit, time to produce the unit, when the unit is available (early game vs late game), effect of upgrades, move speed (golems are pretty poor for offense, for example), and strategy (some units have to be used very differently; for example, the archmage is powerful but vulnerable, so is best used with stronger units to distract enemies).
I don't think a single mathematical formula can be used to gauge balance.