Someone with more experience will chime in, but I think in a rack setup with paper substrate you essentially want the humidity in the room to match what you want in the tub. Maybe you'll get it 4 or 5% higher in a tub. I think if there was a substantial differential there it would be an indication of too little ventilation.

That said I am able to raise my humidity a few percent by moving the water bowl so it sits half on a heated area. I also use 12" tape though, so I have the room to pull that off without restricting a hot spot for the snake... can't comment on your specific rack.