Chocolate Coffee Stout Help

I have a sweet stout in the fermenter right now that I want to add chocolate too. I'll add a coffee toddy to it when I keg it and want to add the chocolate to the secondary. I want a fairly big but smooth chocolate taste and have Ghirardelli Unsweetened Cocoa to add to it. My question is how much to add to the secondary. Only change to the Sweet Stout was replacing the lactose with dark brown sugar as we are all big time lactose intolerant here - so maybe not as sweet but won't make us sick.

Any help appreciated.

Rick near Atlanta

Fermenting:
10 gallons of Apple Honey Cider
10 gallons of Stout

Kegged:
Pumpkin Amber
Pumpkin Stout

Bottled Recently:
Oktoberfest (Dry hopped with Citra)
Winter Wee Heavy