This calculator aims to give an idea of how much of the event you should be able to clear given your stamina gain and the missions you can clear. To use it, set up your stamina gain (some common options are provided as checkboxes), and the quest nodes that you will be farming on.
This is an estimate. Not all variables are accounted for (such as level up stamina, stamina reserves, or already-claimed daily bonuses). In most cases the provided number should be a minimum expected attempts. Like most calculators of this kind, it assumes optimal stamina usage, rather than letting it sit capped.
Daily bonuses (for bonus keys and lottery attempts) are assumed to be completed at the same mission level you select. If the ultimate boss can be cleared, it is assumed to be eligible for the daily bonus (which is strictly optimal due to its cost-reward ratio). Obviously, these cases will not be true for day 1 of the event, so keep this in mind.