Some things I'd do differently next time:
- Include a programming header for the ATtiny85. I had to solder it some 30 awg wire-wrap wire to existing PCB pads to program the ATtiny before adding any other components.
- Use regular 0603 resistors instead of a resistor network. I ended up ordering the wrong size network and had to use discrete resistors anyway.
- Show non-integer voltages by adjusting the relative brightness of adjacent LEDs.
- Indicate voltages above 12V or below 3V by flashing the 12V / 3V LEDs.
- Make it even smaller by using 0.8mm thick PCBs instead of 1.6mm.