Ok, assuming that one wants to maintain their brine at 15%, once the feta goes into the brine and pulls salt out of the brine until it reaches equilibrium, should one start out with a salinity of 30% or so? This is for roughly 40 pounds of cheese in 2-3 gallons of brine. Additionally this will be stored a minimum of 60 days and as I have 200 pounds of it, some will be about 4 months before it's sold.
smiling, great volumes and aging! I think one could do the math (sorry I haven't) to get a ballpark estimate for your volumes/weights with smaller batch and then adjust based on experience.
That said, just FYI, I bought some Feta with whey from large containers (https://cheeseforum.org/forum/index.php/topic,2400.0.html) and measured the stabilized brine (https://cheeseforum.org/forum/index.php/topic,6207.0.html) at 8.5% salt, significantly less than your final 15% which will result in a very salty tasting Feta to your customers.