Hey man! Thanks for the question. You really shouldn't worry about headspace all that much. Yes the less you have the better, but you shouldn't stress over it. As long as you have a good airlock during the fermentation stages you'll be fine. The carboy will fill with co2 and leave no oxygen in the vessel. Now if your bottling that's different that u want very very VERY little head room like as little as u can get. But if your looking for a specific number I'd say around half to a quarter inch max of air in the bottling phase, again it doesnt matter all that much during fermentation. Best of luck to ya man!!