Find voltage across Resistor w/ Op Amp
Since you are using an op amp as a non-inverting amplifier, your gain equation is wrong. Rf/Rin applies for an inverting amp, and you have left off the - sign in your relation. The general configuration for a non-inverting amplifier is 







simulate this circuit &ndash; Schematic created using CircuitLab

So for your configuration (called a voltage follower) the gain is 1 plus (zero /infinty) or 1.