Alright, so my hydrometer is ready, put the probe so it's hovering a little above the substrate. Percentage was off by a couple degrees from the salt test but I simply wrote down the difference (it's one that can't be adjusted). To my surprise when I put it in the tank the humidity was higher than expected, in the high 70's to low 80's in about the hour or so I've monitored it. It's been naturally humid here lately (though where I live conditions change drastically with the seasons) and the setup is in the basement, plus the coconut fiber and my aluminum foil panels.
For the record nothing's actually wet in the enclosure aside from the obvious water bowl (I'll change it out on the day when the snake arrives, just leaving it filled for now to gauge humidity levels). Though I'm certain the levels will go down once I install the CHE when it comes in tomorrow so I'll need to test and keep an eye on that. But for now my humidity levels look pretty nice.