  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 (MESA), we
carried out a series of simulations of nova outbursts, in which the initial CO
WD masses range from $0.7-1.0\,M_\odot$.
  Results. We identified four elemental abundance ratios (i.e. $\rm (H+He)/\sum
CNO$, $\rm (H+He)/Ne$, $\rm \sum CNO/Mg,$ and $\rm \sum CNO/Si$) that satisfy
the conditions for determining the WD mixing fraction, in which $\rm
(H+He)/\sum CNO$ 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 $t_{\rm 2}$ (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.
