Now, as I mentioned in the first post in this series, an ePub file is simply a (very carefully constructed) ZIP archive with a different extension (the three or four letters at the end of the file name). (stillpointdigital.com)
Generally, I take the width that I choose in step 1 and add that at the end of the file name (that's the -800 in the file name above)-- oh, and I take out any spaces and replace them with hyphens or dashes, since ePub doesn't like spaces in file names. (stillpointdigital.com)