Andrew, you sure that wasn't here on the river?! That shot looks very much like here!
I'm guessing the C420 likes a tight leech main? I almost always try to induce a little twist in the 3-5kt range, but at 5-8kts I bring the top batton back to parallel with the boom. It looks like you are getting a wind diversion off the trees on shore, with a wind line to your right continuing up the course. Very similar conditions to here. Do your rules allow outboard sheeting of the jib, or floating jib leads? Barberhaulers on the sheets might help, too.
A couple of the colleges here have some 420's but they still race CFJ's. When I quit teaching (many years ago) the club was just getting into Opti's for the smaller guys because of the growing fleets here. Before that we sailed a local design, the El Toro pram. Very popular boat here in NorCal and Hawaii, but unheard of anywhere else.
I gotta get up there and go sailing with you! I'll buy you a pint.
Evening guys. Getting all the X-mas stuff out and ready here at work, I'm dreadin it. I just despise the holidays.
-Xander