Well actually, if you have 30% Critical there is no calculations to be done.
It's just 30%.
Windmill is one of those "AoE" attacks (Not to be mixed with "Splash") Meaning it has the ability to hit more than one target. When a skill has the ability to hit more than one target, usually that means it wont take any [Protection critical reduction] into account, it'll stay a solid 30% provided you have 30% or higher.
For example, if you were to Windmill a Brown fox and a Desert Dragon at the same time, Fox has 0% protection, Dragon has 50% protection would would be -100% critical chance
It would still be 30% chance to inflict critical.