it may just be my opinion but i tried it all the way soft, half way, all the way hard and ended up going 3/4 of the way hard.

 

the spring in it is for 2 things. One is the sound, which i guess is more of an effect of the other. The second thing that it really is used for would be the strength of the BOV. The harder it is then the harder it is for air to push out of it. So if you are running higher boost then you want to make sure it is a bit harder so that the bov doesnt leak.