Reasonably how much free space should one leave for best performance? (Reasonably because freeing space brings diminishing returns, correct or no?)
Background
I'm testing different SD card schedulers to get the best performance. I'm assuming that when I will have more free space and use the best governor for that situation the performance should be better than now with the best governor for now's situation.
Question
I want to find out if I can somehow find out the best scheduler and read ahead combination without having to test every combination.
So is there some way to calculate/find this? If not then what is the best way?
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.