I believe biscuit malt must be mashed in order to get what you want out of it. That means I would replace your dark dry extract with a pound of pale grain malt. Steep at 150-160F for 30 minutes to an hour (yes, this is a partial mash) and be sure to rinse the grains with additional water. You might need to adjust your gravity up with some additional dry amber malt if you have some.
Ideally, I would only use pale malt extract (dry or liquid) as a source for fermentable sugar as that allows one to control character by addition of specific grains in a specific amount. You may otherwise not know what you are adding when using a darker extract.
Getting rid of some of the crystal malt found in the darker extracts will also help unmask the nuttiness/roastiness/chocolate notes you'll get from using your chocolate and biscuit. About the balance, I'm an all-grain brewer and in my first nut brown I used 10% crystal, 10% amber and 2% chocolate (similar to biscuit). I felt I could have reduced the crystal by half to bring out more of the "nuttiness" of the other malts.
In your case, I think if you just steeped the biscuit malt you wouldn't get the contribution you are looking for. Overall you are on the right track, I'd drink it!