how bout we go with the how many drives to get less than 50% used by parity: 20 drives. 4 sets of 5 drives = 40% of the space used by parity. Capacity for this config = 120 gbs.

as for you the raid 5+6 question...
Capacity: 80 gb
Min Drives: 12
Max Fault Tolerance: 8 drives down at one time, thats two complete raid 5's and one drive from each of the other raid 5's.

am i right? can i have more please? (: