I'm going to be making a 10 gallon batch of pilsner hiding behind some kolsch yeast. I'd like to make 5 gallons of it schwarzbier-ish. I biab so I'm thinking the best way to do it would be to steep some carafa II, boil, and add to the fermenter.

Question: How can I calculate how much carafa/water to make the beer 20-ish srm?

You could always try cold-steeping it in a french press sort of way. This will make your grain even smoother. As to exactly how much you need is a good question. Why not try plugging a certain amount into your pilsner recipe in brewing software and start with that amount. As for how much actual liquid you will need will most likely be trial and error by adding a bit and testing the color to an SRM color wheel. Please let us know what you decided to go with!

PS - Weyermann makes a liquid form of this grain called Sinimar which you can dose into your fermenter directly without all of the pain in the ass.

Well, here's what I'm doing. I'm steeping 10 oz of carafaII in 1 qt of 150 water for about 30 minutes. I cut a piece of voile out to act as a little grain bag. I'll pull the grains out, squeeze the bag probably, and boil for 15 minutes. I'll add it straight to one of the fermentors. Shouldn't be any astingency given that the carafa is huskless so I think it will turn out fine. I'll end up with a little less than a quart to add to a 5.5 gallon batch so it shouldn't be too much of an impact.

I ground it in my little spice grinder. I'd like to try the cold steeping thing but I've got "meet the teacher' night with my son in 2 hours and I want to be done and pitched by then because I don't know how long I'll be gone.

Edit: X that plan. Started the dark grain on the stove to boil and walked outside to check how my kettle was doing. You can guess how that ended up. Painted black stove and charred pot.

I've now got 8 oz steeping in the fridge in a quart mason jar. I'll strain it tomorrow, boil, then add to the fermenter.