The percentage showing (assuming battery life) what exactly is that percentage derived from? What percentage is considered dead-battery territory (ie 3.0V)?
And the screen, is it possible to flip it for lefty control so it’s not upside-down (before I open it up and break it trying)?
No need to do any code or hardware changes.
Assuming you have a later version of the controller you can just visit the configurator (in beta)
It has options for screen orientation as well as setting your own min and max battery voltage to correspond with 0-100%.
Btw there isnt a voltage curve built in so the percentages are just linear. If someone wants to code that it would be great!
Ah ok, the settings were set default to logical things, all I really wanted was to make sure that 0% battery was a safe discharge level. And to flip the screen 
Yup it should read from the settings from the controller when you plug it in.
Did that work for you?
Yes, it did, with one caveat-- by default it loaded showing “left handed” even though it was not, and when clicking on left handed again, it did not switch it. I had to switch it to right first and then back to left. Dunno if this is a fixable quirk or just a very tiny inconvenience.
End result I got what I needed, all good
It appears to have defaulted back to right-handed with no input from me. Does it lose its settings after sitting for a while with no power?
It shouldnt 
but I havent throughly tested that part. It uses dedicated EEPROM storage to save the config.
If it happens again to you it might be a bug.
Yes, it keeps reverting to right handed.
So does my new one. Not sure what to try but messing with the settings anyway
I havent had a chance to figure out the root cause but I built two specific firmwares for left and right handed use thats hardcoded.
You wont be able to change the screen orientation in the config tool but you shouldnt have to worry about it reverting.
See attached files. Just drag them on to the controller drive when its in update mode.
controller-screen-l-r.zip (112.8 KB)
That seems to have worked. I connected with the configurator and clicked the reboot button. Opened the controller like any other USB drive. Renamed the file left hand file from your .zip to CURRENT.UF2 and dragged it over. Instantly the readout flipped to left hand and stayed that way when I unplugged and plugged it back in. Four flights with it upside down I was just getting used to it 
One caveat that some people may run into with this is using a computer with enhanced security that requires encryption to write to a USB device. DON’T encrypt your controller. I had to borrow a computer (my regular one is administered by my employer).