The main rule for smoking is that you never want the product making contact in the smoker. For best results, the product needs even flow of heat, smoke and air around the product. Depending on the size of what you are smoking, this makes the spacing distance kind of a moving target.
At a minimum, you probably want your rack system spacing capable of handling the largest product you ever intend to smoke.
Another option is to devise a flexible system that can be adjusted to suit the dimensions of the product you intend to smoke .