jremington:
For best accuracy you need to measure the Arduino Vcc with a multimeter and substitute that value for 5.0. For example, when powered by USB, Vcc may be around 4.5 V.
For that reason (instability) it might be better to drop battery voltage with the divider to 1volt,
and measure that with the internal 1.1volt Aref enabled in setup().
A resistor ratio of 1:15, e.g. 2k2 and 33k, is good for ~16.5volt max.
Leo..