Neither am I, and I work with this stuff all the time.

You need a statistician and a psychologist at least. The content has to be grounded in accepted psychological terms, and proven statistically relevant.

Maybe you could talk to the DISC people and rent out their expertise to do so...won't be cheap, though. The only other firm I know that has created an assessment tool that really works is Objective Management Group / Dave Kurlan and Associates. You could ask them how they developed their tool and how they proved it was statistically relevant and accurate. If I were in their shoes, I certainly wouldn't give you this expertise for free, though...just be aware of that.