Ok here is a illustration i knocked up for you mate
Dont make the gap between the posts any longer than 8ft it will be to flappy, I worked out that if it is a straight run then you will probably end up with around 7ft between the posts which is good.
Right ....if you use the gravel boards...it will make it easier to level up all the boards when nailing them to the arras rails.... the gravel boards sit on the deck and follow the ground level / terrain, use them mate otherwise it will be a right barsterd to get it looking good as the boards or slats sit on the gravel boards an