Matt
As you get supplies from Butcher and Packer, I assume you are in the US.
The US standards allow 200 Parts Per Million (PPM) Nitrite ingoing in Immersion Cured Products. There are two methods of calculating this:
Method One is used for hams, shoulders, bellies, etc., because it takes weeks for these large items to reach equilibrium. Method Two is primarily used with small items with large surface areas such as pigs' ears, tails, snouts, etc.
Your brisket (assuming it isn't rolled and tied), being similar to (say) a pork belly would appear to be Method 1 for which the calculation is the same as for a pumped product, other than replacing the figure for '% pump' with '% pick up'. As such you could follow the instructions given by Butcher and Packer. The only difference may be the amount of cure that the meat will 'pick up' in 3 days, which is anybodies guess. Because of this I would go for 65gm Cure #1 with the 2 litres of water and 20gm salt in the recipe (although IMO 20gm is not a heaped tablespoon full!). This will give about 195 PPM if the meat picks up 10%, but will still provide a safe level (above 120PPM) at 4.4% pick up, it's the best compromise I can come up with.
Whether the meat will be cured throughout in 3 days will be interesting to see, but it certainly has more chance of doing so with Cure #1 as against saltpetre. It should be noted that the salt in the cure #1 will have an effect on the salt level in the meat, negligible at 4.4% pickup and about 3 times more at 10%. It's so low anyway as to not be of concern.
Please let us know how you get on.
Phil