Ive heard a few folks comment on the 500 having the "best clutch setup" I dont have nor have ridden a 500 so i cant agree or disagree... I can say however that the 400 definitely leaves something to be desired and my 800 too. Mostly depends on what size tires your running, trails vs mud, and rider weight.
Regardless of motor, tires and riding I would try it with no rivets first, if the engagement is too high for your liking. Then try it with the rivets in. Its easier to put the rivets in than take them out.
It all comes down to personal preference... Somebody might make some suggestions that worked for them or that work well for a general starting point, but i would tinker around with it until your satisfied.