I would assume, set your downforce to the lowest settings (least amount of resistance), your height to the lowest (for stability, since you're not hitting any bumps), and all your toe in/out and camber set to as straight as possible. When your toe is set to nothing, you won't be drifting to anyside or your car won't have tendencies to turn. With camber set at nothing, your entire tire can be used to put your power down.
Setting camber to anything else only helps with cornering. When your car rolls through a turn, your outside tire (with camber) will start having a larger contact patch, giving you better grip. So since you are only going in a straight line, you have absolutely no use for camber.