Effective damage is the damage you do when taking average damage and hit rate in account, so for not-maxed weapon it's:

Effective damage = Average damage * Hit rate

Or, more precisely:

Effective damage = ((Minimal damage + Maximal damage) / 2) * (80 + Hit ability)

For a maxed weapon, the average damage is 75% of the maximal damage, and you will hit 90% of the time. So to calculate the effective damage for a maxed weapon:

Effective damage = Maximal damage * 0.75 * 0.9