What's missing in traditional calculators like VicProp is the torque that the electric motor brings to the party. The electric motor can supply almost constant torque across its RPM range and near full torque from the time you turn it on. So there's a mathematical relationship that predicts this power discrepancy and uncovers the false assumption that electric hp can be treated like ICE hp.
Mathematically, horsepower equals torque multiplied by rpm. Or, torque of one newton-meter, twisting a shaft at an angular speed of one radian per second, requires a power of one watt. So power is torque times angular speed (with no extra constants or conversions if you work everything in radians and SI units). So for the same angular velocity (RPM) the electric motor has all of its torque available starting at zero RPM, an ICE does not, and will not until the RPM gets sufficiently high. Thus the exaggerated discrepancies seen at slower speeds.
Given enough time and brain cells one could likely develop a compensation factor for typical ICE hp, but it would have to be non-linear due to the non-linear torque of the ICE. I've seen a 5:3 hp ratio tossed around in the electric car communities, but this does nothing to compensate for exaggerations at slower speeds. Might get you in the ballpark though.
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