Hi simbiotic, there is no right brine % for aging brine in, it's a personal preference. There's some info in our
Wiki: Making Brine article on why higher salt % brines are preferable for cheese longevity and safety. However as you've found, using a very high % brine will result in a very salty feta. If you search the forum on word "salty" you will get several hits on this subject.
The best solution is to soak your feta a little milk before consuming to reduce the salty taste. For next batch you could go with a lower % brine like KosherBaker says, FYI I measured
store bought brine density here.