The 1.8 multiplier is a commonly accepted scaling factor, but the true number needs to be determined on a circuit-by-circuit basis.
The attached LTSpice simulation is a typical 24VAC transformer circuit. It has a schottky fullwave bridge, 1000uF filter cap, and is loaded with a 1 amp current sink.
The waveforms to watch are: node A, node Vout, and the current of V1.
Even though the load current is one amp, the transformer RMS current is 2.2 amps. But the current peaks are over 6 amps.
The actual transformer current to load current ratio is largely a function of load current and filter capacitance. But other circuit parasitics will also play a part.