I'm having a bit of trouble with the humidity in my ball python's tank. I am having a hard time keeping it above 40%

i'm keeping him in a glass terrarium with a UTH covering in in between 1/4-1/3 of the tank, and a 100w heat lamp. covering about half the screen top with towels, Using coconut fibre substrate.

I am trying to mist, but as this is my first snake i'm a little unsure about how moist i can make the substrate. So my question is that, how moist can i make the substrate, and are there any other ways to help keep the humidity up.

thanks