Path of Building does not account for the notable "Eye of the Storm" properly with regard to providing this number.

The Ailment Theshold for most monsters is thier maximum life. However, Sirus et al have a lower Ailment Threshold, which the community assumes is about 25 million.

Check the "Enemy is Shocked" box in Path of Building, but set the shock value to zero.

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In order to calculate what magnitude of Shock we can reasonably expect to achieve we need to answer a few questions:

How likely is it to hit for each possible damage value? How likely is each theorhetical hit to apply Shock? How many times will the boss be hit while the previous Shock is up? What is the heaviest hit which is 95% likely to occur during that window? How much shock will that base hit apply, recursively, as previous Shocks amplify it?

This form takes information about your non-critical damage, attack rate, crit rate, etc, etc to resolve the probability distribution of Shock-applying hits (at no shock) into a system of stepwise linear functions.

Factoring in accuracy and attack speed, we can integrate these functions to find the net probabilty of scoring a hit in that range. Going from the hardeset hit downwards, we find the point where that area is equal to 0.95, and you are therefore 95% likely to score a hit at least that hard.

This 95% confidence hit is then used to apply Shock, repeatedly, until the Shock stops increasing. This is the final number which you will be presented, and can plop into POB.