Quote:
Originally Posted by kiwidude
That was a long sentence 
What the others are suggesting is that perhaps your best bet is just to use the search engine rather than using custom columns as it should give the same effect.
For instance to find all the books that have EPUB but do not have MOBI, type this into the search box:
format:epub and not format:mobi |
And then use the save search tool to the right so the S-O can use the saved name.
Extremely useful with more complex searches
formats:true and not formats:epub <-show me books that need conversion to epub but hide empty books