The approximately 4 ounces of cure #1 per gallon of water is only safe for a short term 10% pump or pick-up, exceeding the 10% or immersing too long would lead to too much nitrite.
10% pump or 10% pick-up mean that the cured meat should absorb 10% of the brine in relation to its original weight. For immersion or pumped products, the maximum in-going nitrite limit is 200 ppm and that corresponds to adding 4.2 oz (120 grams) of Cure #1 to 1 gallon of water.
Thanks very much for clarifying. I'll have to measure the cured tongue next time to figure out my % pickup, but as I added the equivalent of 50 grams Cure #1 to 1 gallon of water, sounds like there is plenty of margin if my pickup was higher.