I'm not 100% sure, but it seems that it grows when it needs the space. In other words, you can't set it to be at xx% left. If you always need more than 5%, you can set the growth to be 10% or 15% and then when you ran out of space, it would grow it. Or you could setup a script to check everyday and when you got to 5% or 6%, grow it manually.
Why do you need 5% free?