LOL, yeah, our gravel comes by the truck load, and they fill our super sacks on site. I have recalculated, and the cost per 1.5 cu. ft. unit is actually much lower than I said. We charge for repacking, shipping, handling, etc. You can get gravel locally, just be sure to put it in a bucket, and rinse it well with a bleach and water solution before putting it into your system.
You assumption on the freeboard is correct, the freeboard is based on the resin height. Our calculation are / .66 to get your resin/freeboard height. Remeber to remove the domes portion of the tank away from the equation. All this being said, it is not that crititcal, it is just the standard practice that allows for a lot of variance in water temperatures, pressures, etc, and the systems will still work almost anywhere in the world without problems.