You should look at the discussion of how to choose your f-stop at this same website. In particular, you might be interested in Paul Hansma's method described therein. He has produced a formula which for a given focus spread on the rail---discussed there also---gives you the f-stop which gives you the best balance between diffraction and defocus (related to depth of field). The formula involves a square root and most people would not want to try using it in the field, but it is easy enough to carry a small table with you. The website has the table. I find the f-stop recommendations too conservative in many siutations, but it at least can give you an indication of the smallest f-stop you might want to consider for a given focus spread.
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