I have started to reply to this a few times, I always stop myself and continue to think about it. It's really a great question, not how much to use... but the use of it... on a higher level.
I really question how much it is used in commercial production, I mean I know Weyeramnn makes it by the boat load. Thats more of a thinking out-loud question as I rub my chin and gaze off into the distance
Anyways, I digress.
I don't think I(personally) would go this route for a Dunkel, the Munich is already going to be very malty from the generous amount of Munich, and you have to be very careful about it not finishing fully. Think full flavored drinkabilty.
That being said, my old recipe was 99% munich II, 1 % carafaII. That was a delicious malt bomb, the mash makes it finish nice and low and dry, but still flavorful and delicious. I recently switched gears a bit on it, and to try something new I modeled a recipe after one of the greats the Kloster Weltenburger Dunkel
I came up with this recipe and its really delicious.
I can certainly see using the sinamar for a schwarzbier, and I would bet money Köstritzer is using it there. But if you are going to use it, I do believe your dosing is correct.
Lets us know!