I think this is one of those you have to experiment depends on the frame size you have and the type of rack you bought.

In my case, I got a Zefal rack and the frame size is small(49cm), so the seat clamp bolt did it nicely and keeping the rack nearly horizontal. I just needed a longer bolt.

I also replaced all the nuts keeping the hardware in place to the locking nuts, so that they don't come loose from the vibration.