Low-noise amplifiers in the radio-frequency (RF) band based on the direct current (DC) superconducting quantum interference device (SQUID) can be used for quantum-limited measurements in precision physics experiments. For the prediction of peak-gain frequency of these amplifiers, we need a reliable design formula for the resonance frequency of the microstrip circuit. We improved the formula for the resonance frequency, determined by parameters of the DC SQUID and the input coil, and compared the design values with experimental values. The proposed formula showed much accurate results than the conventional formula. Minor deviation of the experimental results from the theory can be corrected by using the measured geometrical parameters of the input coil line.