It is base dmg * 2^x, with the assumption your goal is to just have 1 tick...and again, with the lowest roll on duration, we have a ~90% chance for the tick to occur. If you go up to a 55 second duration, you have a 100% chance for the duration to be greater than 3 seconds. It isn't some small chance like you seem to be trying to make it out to be.
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