Roasted grain % in a porter

I want to brew a US robust porter, and initially wanted to use all Chocolate malt for my roast profile. However, after scouring other recipes online, I see that almost everybody uses a combination of chocolate with black patent. Has anybody used just chocolate malt? What are the drawbacks/benefits? What does black patent bring to the table that chocolate does not?

Here is a recipe I just drafted. Anybody have any pointers? I am trying to shoot for a porter that tastes like toasted pistachio skins with a deep bitter dark chocolate character (my girlfriends request)

5 Gallons Recipe
approx OG: 1060
approx IBU: 30

Grains
7 lbs. 8 oz. Maris Otter
1 lb. UK Crystal 85L
1 lb. Belgian Biscuit Malt
12 oz. UK Chocolate Malt
4 oz. UK Crystal 120 L
4 oz. UK Black Patent

Hops
1 oz. Columbus pellets @ 60 mins (12.4% aa)
1 oz. Willamette or Cascade Pellets @ flameout

Yeast
White Labs WLP007 Dry english ale

There are a couple recipes in the back of Classic Beer Style Series - Porter that are a combination of crystal and chocolate malts and no black. So I think you could be okay doing it that way.

Here are some comments I received on a robust Porter I came up with. I haven't had a chance to make it yet.
https://www.homebrewtalk.com/f12/ag-porter-135529/

That's a lot of crystal - you might want to drop the 85 to 1/2lb. Maris Otter will give you a good amount of sweetness anyway, so you don't need that much crystal, IMO. I'm pretty sure black malt is just used to correct the color, which is what it's used for in most British styles. An oz or two of black malt in most any beer will darken it without affecting the flavor. So it may just be a holdover from that practice. I've made porters with it and without it, and I can't say I've noticed a difference.

Looks pretty good to me, but I agree on backing off on the crystal 85. For reference, I like brown porters and for them only use 1/2 lb of chocolate malt, no other roasted malts, and 1/2 pound of a mid-range crystal malt.

That's generally what I use too, though I just roasted some pale to about 120L (roughly) and plan to use a pound or two of that in addition to chocolate and crystal next time around.