Reasonably how much free space should one leave for best performance?
IMHO these governor settings are overrated. Of course it's nice to have such options, but it's confusing users more instead of helping them.

Just check your ROM and ask other users for their opinions. In AOKP for example you only need NOOP and Interactive and you should be good to go for anything.

You should save your time for more important things in Android. Just my advice, though.