Yes you are correct in your
Yes you are correct in your calculations.
Cool down depends on the size of kiln and amount of heat stored. Small kilns cool more quickly than large, fiber lined kilns cool more quickly than brick lined.
When I get a new kiln (6 years ago now) I watch the kiln cool. I look over every 15 mins or so and record time an temperature. Then I can figure out how quickly the kiln cools on its own. For big ones, I pull the observation plugs out when the rate slows dramatically.