Are you working with just the nibs, or are you grinding them to a specific consistency? Pure cocoa will have a very pronounced flavor, so a little goes a long way. I started a small batch with 5 oz of nibs to 1 gal, and finished nicely. However, that was with a lot of experimentation and documentation of results. It really depends on how dominant of a flavor you want. I'm going to assume you want to taste and smell the raspberry, so for a normal 5 gal batch, you may consider 20-28 oz. It may be too much, so I would have to say do some experimentation.
The Brewmaster. Best's Black Forest Meadery. Dayton, OR.