How high does a lower unit (prop) have to be above the keel to generate a rooster tail?

I'm running a Mark 55/ Merc 400 lower unit with a three cylinder Merc on it and have never seen a rooster tail. The cavitation plate is 2 inches above the pad, and the prop is a homemade chopper of 17 pitch (that is not cavitating at this height). All I see is two spumes from the leading edge of the lower unit below the cav plate.

Jeff

PS: I'd post some photos but... What's with this deal? Was always able to post them before.