If you can feed the 3.3V into AREF and use analogReference (EXTERNAL); you will
get ratiometric result. I wouldn't use a voltage divider from the INA121, its dumb to amplify and then
attenuate again, just use a 10k resistor on the output of the amp so it cannot damage the Arduino pin
should it go out of range.
Be careful in your ground wiring not to get a ground loop, that will inject noise into you readings,
follow a star ground scheme (especially if anything high current is running off the same Arduino)