Around 3/4 wet is good for an American Bullfrog, since they spend most of the time in water. Rather than gravel, would recommend something porous to make layer lighter Can find lava rock at hardware store (try Home Depot or Lowes). Also Seachem Matrix works OK but is more $. Above that layer place some plastic window netting, then layer of aquarium blue/white bonded pad, and then the shredded coco. Since frog can splash water out of tub, a drainage tube (clear undergravel filter lift tube with a block of filter foam in end to prevent clogging) going all the way to bottom will allow drainage access.

IRT coco bricks, I get couple gallons after hydrating a Plantation Soil Brick. It would depend how deep the layer is. Remember to use dechlorinated water when mixing them bricks and good luck !.