When I was using a 20 gallon tank, my cool side was consistently below 80 degrees, it always hovered around 74 degrees, even though the hot spot was always at 93. That's why I had a light on the cool end to bring it up to 83.
I don't believe you can guarantee anything since everyone lives in different climates and keeps their own house temps differently.
I have heard the same from other people, some people find it works with just a heat pad or a set up like yours, while others with cool house temps like me need to set it up differently to achieve proper temps.