trying to understand shock mechanics

Note: I'm assuming a crit build. If you don't have reliable crits, skip the steps relevant to crit multiplier and use shocking hits based on your cumulative shock chance rather than crit chance.

Copy the mod "Enemies take 5% increased Damage for each type of Ailment you have inflicted on them" from Yoke of Suffering (or this post, it should be the same) and add it to one of your items, tick another elemental ailment that doesn't influence your dps otherwise, then adjust the value to your actual shock effect.

Now, to see what your actual shock effect might be, you can go for a certain threshold (let's say 70% of your max damage roll for example), multiply it with your crit multiplier, and you have a more or less reasonable magnitude for a shocking crit.

Then, divide this by your expected enemy's hp (e.g. 18328731 for Shaper if poedb is accurate), multiply it by 5, and round down to two digits after 0. That is your expected shock effect for the time being.

At this point you should use the mod I mentioned earlier.

Now you can multiply the number of shocking hits per second (you only need to consider crits if you have any significant investment into crit multiplier) with your shock duration. This is the number of relevant shocking hits to potentially maintain the previous shock effect.

Calculate backwards for the base damage you need to maintain the shock effect you assumed to reach earlier:

Divide the shock effect by 5 and multiply it with your expected enemy's HP, then divide it by your crit multiplier.

Subtract the minimum damage roll from the required damage roll and divide the result by the difference between the max and the min damage roll. The result is your chance to not maintain the current shock effect with a single hit.

Calculate the result of this to the n-th power, where n is the number of potentially shock reapplying hits, and subtract the result from 1. You now (finally) have your chance to maintain the current shock effect. If the chance is unsatisfactory, go back to the start and pick a lower estimated base damage roll for the initial shocking hit.

Unfortunately, calculating a true average shock effect is much more complicated (it involves picking the average highest roll of all potentially shock reapplying hits, and it gets more complicated if you try to include outliers that increase subsequent shock magnitudes). But this should give you an idea of a realistic shock effect for your build. I would personally go for roughly an 80% chance to maintain a certain shock effect. It's likely that your average shock effect will actually be higher, but it would be a fairly reasonable estimate of what you get reliably.