When performing Ohm's Law calculations, what specifies the values for V and I?
V, to me, should be the supply voltage (3.3V or 5V would be what I'm using in this case).
No, V is actually the voltage "drop" across the resistor. Eg, if 1K is tied between 5V and
3.3V, the current I = Vdrop / R = (5V - 3.3V) / 1K = 1.7 mA, and not 5 mA or 3.3 mA.