Shoot Ross a PM for extra details.
I would imagine depending on beer style you would want 30 - 50% headspace - dependant on ferment temperatures, beer type, and yeast health.
On the extreme end, (e.g. a wheat beer with 3068 Yeast or similar can result in a massive Krausen initially.
Maybe an option is to fit a large blow off tube, to allow more space for Wort/Beer.
2c