It turned out that the little el-cheapo boost converters I have laying around are a little too cheap to use out of the box. They wrought havoc with my calibration routine and general accuracy.
Luckily I was able to borrow a 'scope' from a friend and come up with a practical (simple, cheap) solution.
What this converter needed was a filter so I tried just slapping a humongous capacitor on the end. I like to keep things simple. Unfortunately the capacitor on its own wasn't working because it caused the switching regulator to unregulate. It needed a proper coil to make what is known as an LC filter. This can be tailored to lose the high frequency switching noise from the regulator and the mains noise which is picked up over the air.
So I unsoldered a couple of coils from an old broken down router I had lying around and tested those, they worked well, very well. I also tested a coil I made from gaffer tape and 1.2m (4ft) of wire I had on stock. I just wound the wire on the end of a marker pen and glued it together with the tape. It works, not as good as the 'commercial 150uH coil I unsoldered, but well enough to not impact the calibration routines.
The capacitor I ended up using was a 470uF 6.3V electrolytic. Small enough to fit well.
Here's a picture of the hand wound coil I tested. That's the purple and black round thing above the center of the LCD screen. You can see the 'scope indicating a ripple of about 4mV which is actually really good. This big a coil will aggravate the 50/60Hz mains hum though, but the capacitor will handle that, no sweat.
To be honest I ended up using a 150uH coil I unsoldered from the old router because it was much more compact and works a tiny bit better. But this will do in a pinch!
It also helps if you mount the boost converter physically farther away from the sensors and Arduino. I ended up sticking it in the same place anyway, because of space constraints, no biggie.