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AIberto - if you're adding back issues, try adding them in batches for a single periodical (ie all the PC Worlds, all the Bytes etc) then you can select everything in a batch and use Bulk Edit to set common fields.
FWIW
I have a separate Journals Library, where the Author Name is the Periodical name (eg Foreign Policy) and the Title identifies the issue, most often its the date of publication eg 2010-11-18, some have an issue number, some have Season eg Winter 2011 - which I 'normalize' to 2011-Q3
When adding new issues I limit the Library list to the periodical in question and enter the metadata directly into the library list cells - then I can see what I have previously entered for that periodical. Since I started doing that for periodicals I now do it for everything, so that I rarely use the single book edit metadata - when I have a metadata source I hit 'W' on the selected book(s) in the Library list.
BR