@Buck47, never thought about that, I guess whey has a very slightly different Specific Gravity to fresh water but that would make very little different to the final % brine. Whether a small difference in % brine makes a difference I don't think so as people use generally anywhere from 16% up to saturated.
@ryan, from the page linked above "% salt is a measure of the weight of salt divided by the weight of brine, i.e. if you dissolve 1 kg of salt in 5 liters of water, the brine will weigh 6 kg and you will have 1/6 = 16.7% salt brine". So for your mix you would have made a 20 ounces weight of salt / (64 ounces weight of water + 20 salt = 20/84 = 23.8% brine. 31.25% is impossible at standard room & temp, max is ~26% salt at 60°F/15°C.
From the table at the bottom of that page if you want a 16% brine (the minimum for minimizing unwanted bacteria growth and will give you the lowest salty taste) then use 1.586 pounds of salt in 1 US gallon of water or preferably whey.
Feta cheese are meant to be ripened (OK they don't change a lot) or stored in brine, not temporarily bathed, see
these pictures. The plastic bagged feta are repackaged from the large containers where shipped in brine, pictures further down that thread are of old Greek store where stored in barrel and large containers in brine. I also keep mine for months in brine until I use it all up.