Context. Classical novae are powered by thermonuclear runaways occurring on the surface of accreting white dwarfs (WDs). In the observations, the enrichments of heavy elements in nova ejecta have been detected, indicating a mixing process between the accreted matter and the matter from the outer layers of the underlying WDs prior to nova outbursts. However, the mixing fraction in classical novae is still uncertain. Aims. The purpose of this article is to investigate some elemental abundance ratios during nova outbursts that can be used to estimate the WD mixing fraction in classical novae. Methods. By considering different WD mixing fractions with the stellar evolution code Modules for Experiments in Stellar Astrophysics, we carried out a series of simulations of nova outbursts, in which the initial CO WD masses range from 0.7-1.0 M-circle dot. Results. We identified four elemental abundance ratios (i.e. (H + He)/ Sigma CNO, (H + He)/Ne, Sigma CNO/Mg, and Sigma CNO/Si) that satisfy the conditions for determining the WD mixing fraction, in which (H + He)/ Sigma CNO is the most suitable mixing meter. We also estimated the WD mixing fraction in some representative classical novae. Additionally, we found that a higher metallicity (i.e. higher WD mixing fraction) is preferentially accompanied by a longer t(2) (the time of decline by two magnitudes from peak luminosity) during nova outbursts. Our results can be used to constrain the mixing process in classical novae.