This is what I have discovered:
Forum dependency works, but not as a %.
eg, I have my forums rated on a scale of 0-2. on the gradebook this shows up as 0-100%. But in the certificate module you have to ignore the % sign after the grade and assign the required grade as 2%, i.e. 2.
I looked at the code in the lib file, and there are checks about the rating scale, but I don't really understand them, so I left it alone.
Bottomline: Test your dependencies with different values, and ignore the % sign.