10 lbs of Maris Otter it is!
I guess I'm up then.
I've been mulling this one over.
Robust Porter style works well with some grainy, malty flavor and aroma with some sweetness. Ruby and garnet highlights in the color are sometimes present. A higher kilned malt that provides the grainy, malty flavor and aroma, with some added sweetness fits well with the style. Adding the beginnings of some of the Ruby and garnet highlights, will give some options to the other grains to be added after this, giving them some flexibility in their choices.
My suggestion is to add 1.5 lbs of Munich Malt to the mix, with an SRM of 9 as found in German Munich Malt. This will fit with the Robust Porter style of an English version, as that would be the most likely used Munich Malt back in the day. I don't think a U.S. Munich Malt is the best choice, although the U.S. 10L and 20L versions wouldn't be in appropriate.
I use a lot of Munich Malt and find that it adds a deeper malty flavor to many of my beers. I use it when I want to the maltiness and a little sweetness too. I also like how the orange-amber color that it imparts, intensifies the colors of other grains (such as crystal, Black Patent, Chocolate, and Roasted Barley).
As far as the percentage of Munich Malt in the total grain bill, I think 8.5% to 10%. I'm leaning towards the 8.5% in my mind.
Thoughts? Suggestions?