According to the Hitchhiker's Guide to the Galaxy, 42 is the answer to Life, the Universe, and Everything.
1000ppm (0.1%) may be good enough. But I still want to look into hardware timer interrupts. This project will be growing in stages. The Pro Mini is fine for now, but I will need an Arduino Mega eventually. And the Mega will have to run at 3.3V 8MHz (ATmega2560V on a custom PCB) so I will eventually have to monkey with the bootloader and lots of other under-the-hood stuff. This is going to be a long learning experience.