I know Melanoidin Malt can be used as a substitute for decoction mashing, provides the richness expected in certain styles, and may improve red or brown color. So at what percentages does the malt achieve these results?
First of all, in order to think that melanoidin is a sub for decoction you have to believe that decoction has an impact on flavor. My own experiments, as well as those of others, do not support that. Melanoidin will boost the maltiness of the beer in a kind of sweet, fruity manner, as well as have an impact on flavor as you describe. Too much of it will make a beer taste off, almost with an oxidized caramel flavor. I would say keep its use well under 10% of your total grist. If you you haven't used it before, start at 5% and see what you think.
Five percent of your grain bill is generally a good starting point for Melanoidin Malt contribution. Be cautious using more, as Melanoidin Malt has a very powerful and distinct flavor.
I think I've also seen a comparison somewhere on the internet between decoction mashing and Melanoidin Malt, so maybe Denny will weigh in on this one...