Looking at the calculator, if I put R1 = 5000 and R2 = 10000 then the output voltage is 8.533 -- wouldn't this kill my Arduino ?
What you think? If you were just told NOT to go over 5V?
Dividing voltage is easy with resistors. If you have 12V to divide, try this:
Connect twelve(12) 1k resistors in series, and measure voltage between each connection and GND? You should see the correlation between voltage and the any resistor combination. That way you can calculate any voltage to input pin. Works with output too, but then you need to take current limitations into your calculation, output pins can't handle much, safely 20mA.