Hi there and welcome to TorqueCars. It is really hard to get application and car specific setup advice that is accurate. Presumably you've done this before?
I have to reiterate that a cars suspension is setup for the driver preference. As it's tarmac I would say you'd get away with going a bit lower than you would for the rough stuff.
Quality components is essential when it comes to competition cars. Any weakness will be evident by failure of the component when fully stressed.
Where is the rally? Where are you from? What is your current suspension setup? What is your budget?