Correctly sizing a battery bank takes a little more care than that, Angus.
Consider the battery to be a vat of water that gets filled at occasions, and gets drained at other occasions or continuously.
The trick now is to have such a size vat that it never goes below 50% full. If it gets constantly filled up you can use a small vat, if it gets constantly drained and only once a week filled up you need a larger vat. Etc.
The water analogy is a simple but illustrative one.
The correct vat size (battery bank size) is also influenced by backup charging measures, like a gasoline powered genny when there's no sun or wind.
So, in short, one needs to make an analysis of the situation. Normal usage, the situation it will be used in, etcetera. General rule of thumbs like C/20 have only one advantage; that they're simple...