Heat milk slowly to 93°F. Dilute calcium chloride in 1/4 cup water and mix in.
Sprinkle C21 culture over the milk, rest 2 minutes, mix in, and ripen 60 minutes.
Dilute rennet in 1/4 cup non-chlorinated water, mix in for 1 minute, and set 40 minutes.
Cut curd into 1/2-inch cubes. Gently stir 20-30 minutes while maintaining 93°F, then settle 10 minutes and remove whey.
Transfer curds to a basket mold, add follower and 1 lb weight, and drain 8 hours at 68-72°F. During the first 2 hours, unmold and turn frequently.
Cut into roughly 1/2-lb pieces, place on a draining mat, sprinkle with a small amount of salt, and drain 6-12 hours.
Brine in saturated salt brine for 4 hours. Remove and dry 1-3 days at 48-56°F under loosely draped butter muslin or cheesecloth, turning several times daily.
For storage, make an 8% brine with 6-8 oz salt per 3 quarts sterilized water. Cover the feta with brine in a container with minimal headspace. Store at 48-50°F up to 30 days, then 45-55°F for further aging.