Trying to establish a formula to figure out how many enemies should I load per level?

Variables:

Total Game time (sec)

Tower DPS

Tower Range

Enemy HP

Time Tower deployed (sec) = total Game time - Avrg Deployment time for all Tower.

Percent of Map in range = Total range area X 100 / total Level surface.

Actual Total Tower DPS = Total Tower DPS X Percent of Map in range.

Total HP for all enemy = Time Tower Deployed (sec) X Actual total Tower DPS

This does not take into account overkills so an arbitrary multiplier value should be added … .

Am I overthinking this?