In my experience, you'll find the best slug accuracy with somewhere between 0.015" and 0.005" of choke.

In an ideal world, with as much choke as you have, a guy with a Sunnen type hone could "regulate" p.o.i. at the choke.

Or you could start with one of these: http://www.gunsinternational.com/Manufrance-Robust-12ga-Shot-Ball-BLNE.cfm?gun_id=100222159

YMMV