make a current transformer
If you want to do calculations, you need at least an inductance factor (AL value) for the core. Although ferrite torroids are not optimal suited for LF current transformer, they may be used, even with a low permeability (≈ AL value) core, possibly with less performance.
Typical current transformer could be 100:1 up 1000:1, implying a 100 to 1000 turns secondary winding. The secondary winding could either be connected to a load resistor or a current amplifier with virtually zero ohms input impedance. The latter would give better accuracy but most likely isn't necessary for this application.
With increasing load resistance, the transformer LF cut-off frequency will rise. It should be, however considerably below your 50 Hz operating frequency. A too high load resistance could also cause core saturation, possibly excessive core heating and high voltages a the secondary. If have no core data, you can start with a resistance that would result in e. g. 100 mV secondary voltage (Uload=Iprim/n*Rload).