Your cure amount is on the high end, but acceptable. I prefer a lower strength cure, and longer time but that is a personal preference. A couple of changes you might consider is to not boil your brine, only simmer it. And the 3 hours is quite a long time.... 1 hour works well. I inject brine into all my wet cured meats over 1" thick, this way it cures from the outside in, and from inside out. Another tip is to wait until you cool the brine down before adding the Cure #1. The is a lot of debate on how heat affects sodium nitrite. Many labels on Cure #1 actuall mention cold water. So keep that in mind.
You can probably stop reading right here because with a couple of changes.... what you are doing will get the job done.... but this is how you can check other recipes you find online, or just prove to yourself how a brine cure works.
The way Cure #1 is calculated in a
brine is somewhat easy. You weigh the meat, weigh the water and then calculate the amount of Cure #1, which is 0.25% of the meat and water weight. For example you have a 6 pound brisket and will use 1 gallon of water for your brine. Converting to grams, this is 2724 grams of meat, and 3786 grams of water for a total of 6510 grams.
Next calculate the Cure #1: 0.25% of 6510 equals 16.3 grams of Cure #1 which is a hair over 1 tablespoon. This concoction will come in around 156 parts per million of sodium nitrite. You had 6 quarts of water, so the Cure #1 amount would be a little over 20 grams.
That said, look at the heavy tablespoon as the minimum amount of Cure #1 in one gallon of water. More can be safely used, in fact the maximum amount of Cure #1 in a gallon of water is almost 4 tablespoons BUT you are now pushing the 200 parts per million on the sodium nitrite, which is the maximum recommended amount. So.... most curing folks will agree that 1 to 1.5 tablespoons of Cure #1 is the minimum, and 3 to 3.5 tablespoons of Cure #1 is the maximum when making 1 gallon of brine.
I suspect commercial operations go heavy on the Cure #1 so they can shorten their cure time. Many will inject about 10% of the brine, which also shortens the cure time (I inject any meat over 1" thick).