Pretty elaborate calculations... But that's not including the fact that some proteins are not digested at all. Like the corn... That would lower it, too (I think it's far less than 1/2 digestible). I think it would bring science diet below nutro...
I did one of those for Iams a while back (post 19):
http://www.pets.ca/forum/showthread.php?t=15049
I tried to deduce how much of the food's protein came from sources that were not useful to the dog... I used the canadian food breakdown website back then for the nutritional breakdowns.