I normally rack to secondary when the primary ferm is still barely going (about one bubble every thirty seconds). That way the secondary will have a bit of a CO2 blanket. But I don't think that it's really necessary with beer. I haven't heard of people worrying too much about head space in secondary, probably because it's usually only there for two weeks or so. If I were you, I'd rack it to secondary when primary is done and quit worrying.
BTW, welcome to the forum!
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Primary: Empty
Secondary: Cyser
Bottled/aging: Robust Oatmeal Porter
Bottled/drinking: Hammerhead Clone Attempt #1
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