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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
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Fortunatelly we are wikipedia gifted :-) Read the first paragraph and "Change of base":
https://en.wikipedia.org/wiki/Logarithm
At the end logarithm to the base of 100% - evap_per_day (in %) of thershold (in %) gives you the number of days.