Fun With Monitor Calibration

I've been using a defunct Pantone Huey Pro to calibrate my display, for pretty much as long as I can remember. While the device hasn't been officially supported for a while, it had continued to function until recently. Since El Capitan came out, the software hasn't been working properly, and my display has been slowly drifting. On top of that, somewhere along the way, the gamma got screwed up, which I didn't realise. So the upshot of this is that my screen was too bright, and so I was outputting my images too dark.