In a forward converter, the primary current is the sum of the magnetizing current \$I_{mag}\$ and the reflected inductor current \$I_L\$ which, on average, is the output current \$I_{out}\$ scaled by the transformer turns ratio 1:N. This current circulates in the primary side during the on-time duration. The below drawing shows a typical waveform excerpted from a report I posted on the over-power phenomenon in the forward converter:

If we neglect the magnetizing current circulation, it is possible to approximate the triangular waveform into a square waveform and easily calculate the average input current. Average calculations are about area determination. Look at the below picture to see how to simply determine the average value by stretching the original area along the switching cycle:

Assume you have a forward converter delivering 5 V 10 A with a transformer turns ratio 1:N=1:0.16. The input voltage is 100 V and the duty ratio is 31.3% (neglecting all the drops). The below Mathcad sheet shows the input current to be, on average, 502 mA:
