In this paper, an analytical model has been developed for improved assessment of Miller capacitors for high-frequency metal?semiconductor field-effect transistors. Depletion layer underneath the Schottky barrier gate has been divided into four distinct regions, and by evaluating the charges associated with each region, gate-to-source ($C_{GS}$) and gate-to-drain ($C_{GD}$) capacitors, {commonly known as Miller capacitors,} have been defined accordingly. Mathematical expressions have been developed both for the linear as well as for the saturation region. Miller capacitors and their variation as a function of applied bias have been assessed. It has been shown that the proposed technique offers better accuracy in determining the Miller capacitors, especially $C_{GD}$ of the device relative to other reported analytical capacitor models. This improved accuracy has been achieved by involving the entire Schottky barrier depletion layer piecewise for the assessment of charges defining the Miller capacitors. Thus, the developed technique could be a useful tool in assessing the AC response of the device with more precision.