So data should not be based on averages? what should it be based on? This is the average for Bangui.
Bangui, Central African Republic
The warm season lasts from January 23 to March 18 with an average daily high temperature above 93°F. The hottest day of the year is February 9, with an average high of 94°F and low of 69°F.
The cold season lasts from June 20 to August 27 with an average daily high temperature below 87°F. The coldest day of the year is January 3, with an average low of 65°F and high of 91°F.
The hottest day average low is 69 and the coldest is 65. That would put the average low for the entire year at 67. Yaoundé, Cameroon average low for the year is 68. The average low for the year in Khartoum, Sudan is 73.5. The average low for the entire year in Tambacounda, Senegal is 72.5. I know what you are going to say. Well those are the low averages you need to average them with the highs. That would be true if they only came out during dusk or dawn only. If they go out at night though the lows are what to look at. It gets in the 90s and 100s during the days in some of these places but they stay put under the ground in these temps for the most part.
I do understand humidity. Your argument though is based on how humidity works in general. We are talking about a tank. When the humidity in the air is less then what you are trying to maintain it isn't so cut and dry. How much water surface is in the tank? How much misting is going on? Is the top covered fully, partially, or nothing at all. You can't debate something without all the info.