Here's a nifty little program that I came across, It will calculate depths of sand,coral,etc. You can download it from my webpage.
It seems to be accurate for Southdown sand anyway! Give it a try...
for 29 gallons i would recommend thirty to fourty pounds. U really don't need that much but I would probably buy fifty pounds to be safe. Use a bag at a time and each time stick ur finger in. Usually u want it one in a half to two inches, but the more the merrier...