What is your 8 mV voltage source? RF signal generators with a 50 ohm output port have a 50 ohm internal source resistance and if there is a calibrated output level, that output is with a 50 ohm external load, so open circuit it will be twice the indicated output. If there is internal source impedance, it is in series with the external resistance of? test circuit, making your R2 value in the calculations 100 and 150 ohms. Assuming a high impedance measuring device for the voltages, the internal and external resistances won't cause significant voltage drop, making your Vin voltage measurement OK. Audio and function generators may have various output impedances, including very low internal source impedances; you need to know what is going on in your source generator to get the math correct.
If you want to include an evaluation of reactance, you could use an oscilloscope to synchronize to the source generator and then measure the phase shift on the other channel. The math gets trickier to add phase angles to your equations, though.
