Through a lot of figuring and searching, and with the suggestions made for figuring, I have figured that to cover 560 square feet, I need about 63 cubic feet of plaster, so about 21 cubic feet of lime...
then figuring based on a figure I found for 30 pounds per cubic foot of hydrated lime, I figured I will need 620 pounds of lime, which comes in 50 pound bags so I need to buy 13 bags of lime. By Roger's suggestion, this will yield a plaster thickness of about 5/8" (I used 3/4" for my figures)
