Im happy to help you out.
Just for curiosity, why are you nervous to invest Ina heat pad?
Are your lights regulated by a thermostat?
Unfortunately, those analog thermometer/hygrometers are wildly inaccurate. They can be off by as much as 15° or 15%. It's definitely worth picking up a few digitals.
For keeping humidity up in glass it's very important to cover the screen top. I actually wrap my glass enclosures in 1/2" foam board insulation. I cover the top, back and sides. This helps immensely with heat and humidity control. I live in MA and winters here get cold and dry. Since I started insulating the enclosures I never need to mist anymore, maybe when they're in shed to bump humidity, but not in general.
In a 40 breeder you've definitely got room to add a few more water bowls too.