My trays are aprox 16"x7", and I use 4 cakes per tray. I heard some where that your mycelium layer should be around 1 1/2"-2" thick. I like your idea of casing each jar in its own patch in a large tray, leaving a little space between each. Seems like that would maximize your yeild. Take your total dry weight and divide by the number of cakes you cased and see what your dry yeild was per cake. Compare with the average yeild. Keep track of these numbers everytime you change your technique you'll learn what works, and how to maximize your yeild 
The Lizard,
[This message has been edited by Lizard King (edited January 15, 2000).]