Well you want to shoot for 50% normally and 70% during a shed. Will 40% be bad for your snake? probably not, but he will probably have a hard time shedding. You could try up humidity with some damp spaghnum moss. If you have a screen top, you can cover 2/3 of it with a layer of tinfoil and then a layer of duct tape. This will prevent some humidity from escaping through the screen.
Is your build in 50 watt bulb built INSIDE the cage or is it mounted somewhere where the snake can't get to it?