George, don't panic. You are correct that keeping it in the case will mitigate the rate of humidity changes, which are also an issue in addition to long term absolute indoor humidity. Many folks live in climates that are moderate for humidity. Worry if you live in the desert southwest with major air conditioning use all summer or in very cold areas (Alaska, Canada, Norway) where there is serious winter heating. Either heating or AC will dry out the indoor air to problematic levels. If you get zapped with static electricity around the house or are having sinus issues from dry air, your guitar is probably not comfortable either.
In Alaska or Arizona, I have to pay close attention to indoor RH. My house in Alaska had a real hard time getting up to 20% RH in the winter, and we put 8-10 gallons of water into the air every winter day. It but was fine all summer. When we visit Arizona, they think 15% RH is positively damp.
I'm a pilot, so temp versus dewpoint is an indication of when the airborne moisture will condense into fog / instrument flying conditions. I've never tried to calculate the indoor RH from that data, but it is surely possible to do so.