I would agree with Stinky but add Flora Danica. It has a ton of uses and adds a buttery taste to your cheese.
I've made Feta a couple of times with Choozit's Feta B culture following the guidelines in Caldwell's Artisan Cheese Making. The goal pH following draining is 4.6-4.8. Caldwell mentions that this should take up to 12 hours. I hit the goal both times in 11-12 hours at about 4.78. The first batch was with 2 gallons of vat pasteurized whole milk. The second was with 2 gallons of P&H milk. Other parameters (temperature, ripening time, cut curd size, etc.) were pretty much the same. Yesterday I made a third batch using one gallon of each of the above milks. Caldwell's recipe states that Flora Danica can also be used. So, I opened a new un-expired packet and used FD. Being a degreed and somewhat anal chemical engineer when it comes to these kind of things, I pretty much stayed the course following the same parameters as in the earlier batches. Being of this disposition I go through pH buffer solution dialing in my pH meter about as fast as a wino on skid road goes through Thunderbird wine. After 12 hours the pH was still about 5.1 so I let it sit until about 11:00am this morning - a total of 20 hours of draining. The pH had leveled out at 4.92 - the same as it was when I checked it six hours before.
This could be a result of the different cultures. But, I made one other change that might have affected the pH drop but wonder if it could have affected the ultimate pH: The recipe called for cutting the curds at 3/4 to 1 inch. I had used my three quarter inch cutter the first two times. This time I used my new 1-inch cutter - the one that cuts in two directions at once and looks like you could play ping-pong with it or cricket if it were larger.
So, what caused the difference in ultimate pH - the curd size of the starting culture?