I did a Chocolate Milk Stout last year, based it off of a Left-Hand Milk Stout Clone recipe and added cocoa nibs to the secondary. I did end up using exactly 1lb of lactose sugar, and it turned out perfectly.
I think what you may want to keep in mind isn't the amount of sugar so much as it is not using too much cocoa... personally I'd recommend soaking the nibs in alcohol, and then using a graduated pipette to scale tiny additions from a single glass up to the whole 5 gallon volume. Too much cocoa nibs will result in an intensely bitter/dark chocolate flavor, which very well may end up masking your intents for this to taste more like chocolate milk. In my personal experience, anything over 1lb of lactose sugar in a 5 gallon batch ends up being too sweet, though your tastes may differ and there's nothing wrong with adding an extra .25 or .5 lbs if that yields the flavor you're looking for.