Recently, there has been renewed interest in electro-optic time lenses due to their favorable properties for quantum photonic applications. Here, we propose a new analytical approach to estimate the chirp rate of a time lens implemented using an electro-optic phase modulator driven with a single-tone radio frequency voltage. Our approach is based on a user-defined time aperture for the time lens. We observe that the temporal aberrations depend on the amplitude of the phase modulation and the effective time aperture of the time lens. We derive an analytical expression for the maximum phase error that will allow the user to choose the maximal aberration-limited time aperture of the time lens. We apply our formalism to a Fourier-time lens system for the spectral compression of Gaussian pulses and find the optimal parameters of the setup. Our approach will provide a handy tool for efficient temporal system design based on electro-optical time lenses.