20 is fine. Just always keep a side covered. Since Heat Rises her belly isn't going to be as warm as needed. I normally keep my humidity gauges on the side without an UTH(under tank heater). None of my 6 BP have basking light. I never really understood where the basking light started, they are in mammals hides most of the day in the wild.
If you feel she is already very stressed out. Don't push her. So long as she hasn't retained her eye caps. Placing her in a warm bath tomorrow and letting her roam her tank with the soften shed (for me) always does the trick. maintaining gradient heat & proper humidity is a balancing act in the glass tanks. So really keep up on it and get a UTH if you don't already have one.