The two big numbers I use when brewing are ratio and time. With these set correctly, you can do batches from 1L to 25L and the only difference is how hard it is to drain/filter.
I cold brew large batches to keg with nitro and also drink it straight and here are a few notes I have about my two numbers.
The time range everyone says is 12h-24h. I think this is about right and I think the sweet spot for normal brews is about 16h at room temperature. Closer to 24 hours can often taste "over extracted" and will be more bitter, but also more caffeinated. If you're trying to brew stronger, I'd try keeping your ratio the same and adjusting time to see what you want the brew to taste like first. Higher ratio brews seem to also benefit from longer steep times as solubility can become a factor, so usually if I like a coffee at 0.125 ratio for 14 hours and I go to make a 0.200 ratio brew, I'll steep it for 15-16 hours instead to make sure I can dilute it down to how I like it. This is where things tend to be by "feel".
Ratio is much more scientific. It's just how much water and how much ground coffee you use. Protip: Use metric units and measure your ratio as Kg coffee : L water. This makes life way easier since you don't buy water by weight, but 1 Kg of water = 1 L of water, so your ratio makes sense. Here are the ratios I regularly use:
- 0.100: "Beginner" Nitro cold brew. If you want to serve a keg of cold brew and nitrogenate it, but your audience is family or non-coffee enthusiasts, this is a good ratio so that people don't chug a cup of it and feel like their eyes are going to pop out from the caffeine.
- 0.110: "The book" cold brew: loads of people swear by "1 gallon of water per 1lb of coffee" ratio. That's 0.110. I rarely use it, but here it is for reference.
- 0.166: Flat, everyday cold brew, fresh. I frequently make fresh cold brew overnight for myself. When I wake up it's room temperature and when it drips through a filter, it has a pleasant CO2 frothiness to it. I drink this over ice, which dilutes it close to 0.125 and if there are leftovers, I'll store them in the fridge and drink them straight later. It's a good ratio if you're not planning anything and want it to be flexible.
- 0.185: Strong. When I make nitro for enthusiasts, I brew here. This is a really rich, strong flavor. The creaminess of the nitrogen makes up for the fact that it's not diluted. Make no mistake, this is very strong to drink straight.
- 0.200: Concentrate for 1:1 dilution. I don't typically do water dilution. I rarely brew here, but this concentrate could work with a mix of ice and condensed milk or just cold cream.
- 0.250: Strong concentrate for 1:1 or higher dilution. If I make concentrate, I make it at this ratio for 16h+. Again, I don't do water dilutions, but over ice and cream or condensed milk, this can really make a good coffee flavored beverage or save space for storage.
Right now, I like Ethiopian beans for flat cold brew. I've tried several beans for Nitro and not had one I didn't like.
Hope that helps!