Humidity is a difficult discussion without straying into Nerd World.
Goal Number One! Get a quality Temperature /Humidity device, digital!!! I recommend at least two units, that require being plugged in or batteries, as most Homes vary wildly.
I have seen a number of documents and writings that state this or that as the best relative humidity (RH) for Parrots. IMHO, they are two high for the average home to first maintain and second, not have serious problems with window sweat and/or mold. Having lived in a huge home that used hot water heat and was 100+ years old and exchanged air like a barn, maintaining a consistent RH was impossible. Now with a newly built home, consistent RH is not a problem, but quickly obtaining window sweat can be an issue.
Since the membership live in both half's of the World and a wide variations in temperature zones, there is likely no perfect system, but below is a good starting point.
The Humidifier: Use a cool water system that uses a fan, filter and water tank. Look for one that has a RH setting on it and adjustable fan level -- because it is just easier to use and takes the guessing out of the equation.
RH and Your Home or Apartment: Whatever setting you start at, watch for window sweating (steamy or droplets along the glass base as it contacts the window's frame). That is your room or home's highest setting.
What I found that works for us: First we are located near the 45th parallel, which means that we commonly start heating in mid-late September and end sometime in late April or early May. Each year varies widely as does the Weather! We target an RH around 45% based on our Home, Parrot and People. The 'perfect' number for your home will vary depending on the quality of the measuring device (remember digital unit) and its location in the home.