Keep in mind that the more spawning areas you provide...the more spawning you will get so you have to determine if more spawning suits your goals. My pond is more of a hybrid hole of water with HBG and HSB and I only put in about a 144 square feet of gravel in my 1/4 acre pond for the HBG. I an definitely getting recruitment, but I can't tell if it's too much or not enough because my 2 year old pond has not settled in yet. I would suggest starting off with a smaller portion of gravel and adding more if you feel you need more forage production as the years pass.

My gravel bed starts at the bank in about 12 inches of water and extends out to about 3 to 4 feet of water. I put it a little further out than the 36 inches you mentioned to account for any water level changes in case my pond's full pool fluctuated during the spawning times. I also used regular ole' creek gravel that ranged in size from sand to baseball size rocks...it was easily moved from the nearby creek prior to the pond filling up.

Fish on!,
Noel