The FEM modelling combined with the experimental nanoindentation hardness and indentation modulus depth profiles with the indenter tips of different radii on hard W-C coating/softer steel substrate with different coating thicknesses, revealed the strong influence of the tip radius and coating thickness on the shape of hardness and indentation modulus depth profiles, and the subsequent underestimation of coating property values extracted from the profiles. The effects of tip radius and coating thickness on hardness profiles have the same physical reason - faster penetration of the zone of plasticity into substrate at the same relative indentation depths when the indenter tip radius increases and/or coating thickness is reduced. Thus, the validity of the "10% rule" is limited also by the indenter radius. The main parameters critical for proper nanoindentation measurements in the studied W-C coating/steel substrate system include indenter tip radius <= 500 nm, coating thickness >= 0.5 mu m and the ratio of the tip radius to coating thickness <= 0.1.