If the DC coil resistance is about 5 Ohm, it will be better to have the series resistor the same value for max accuracy. For reliable result, you will need a meter with 4 digit or more display.
48V AC is perhaps too high; you should use a voltage so that the total current (same current flows via the resistor and the inductor; they are to be put in series) is around 1A (higher current will give greater voltage drops but unnecessary heating; you have only the voltage to be measured).
When you use the equation, you must remember that your L is actually a L-R (apart from the extra R) combo.
I cannot guess how you got 80 mH. Please tell me.