As an approximation, the current is the vector sum of an inductive magnetizing component (Inoload= no-load current) and a resistive load component (Iload).
Itot=Iload + j*Inoload
For powers near to nameplate power for high-speed motors, the ratio of actual current to nameplate current is reasonable approximation of the fraction of full load, since Inoload will be somewhat less than Iload and it’s contribution is reduced even further by the vector addition.
At lower powers, or for lower-speed motors, the magnetizing component becomes more significant. If we want a more exact solution, we can estimate:
Iload=sqrt(Itot^2-Inoload^2)
P = Iload/Iload100 = sqrt (Itot^2-Inoload^2)/Iload100
where P is the fraction of full power
Iload100~Inameplate but more exactly Iload100=sqrt (Inameplate^2-Inoload^2)
we have neglected changes in power factor, and efficiency.