Cure salt is fine grained and denser. 1 tsp. of cure #1 salt will weigh more than 1 tsp. of kosher salt. So it will not be an equal exchange by volume using measuring spoons. Best way is to weigh the cure salt, then subtract that amount of kosher salt.
Cure #1 is 93.75% table salt so I will use the weight given for table salt in my reply. The weights will be very close.
1 tsp. of Kosher salt weighs about 4.8 grams while 1 tsp. of table salt weighs about 6 grams.
So, you would need to remove 1 1/4 tsps. of kosher salt for each tsp. of cure you add.
Someone else can check the math, but I'm pretty sure it's correct.