Fermenter head space

After introducing bugs into secondary how much head space is necessary to accommodate fermentation with bugs as well as pellicle?

My plan is to ferment lambic with call ale yeast and then transfer to secondary where bugs will be introduced?

 

you don't have to make plans for a pellicle, it may or may not form and it does not have much bearing on how the beer will taste or develop. that said, i fill the carboy up near but not right to the neck.

 

Was wondering this myself... Have a small secondary experiment going in a growler filled very close to a stopper/airlock, and a few oak cubes floating in that small space. I added the dregs of a JP Bam Biere (brett, lacto, ???) so I'll see how it goes with very little headspace, plan on giving it at least 3 months.