  Classical novae are caused by mass transfer episodes from a main sequence
star onto a white dwarf via Roche lobe overflow. This material forms an
accretion disk around the white dwarf. Ultimately, a fraction of this material
spirals in and piles up on the white dwarf surface under electron-degenerate
conditions. The subsequently occurring thermonuclear runaway reaches hundreds
of megakelvin and explosively ejects matter into the interstellar medium. The
exact peak temperature strongly depends on the underlying white dwarf mass, the
accreted mass and metallicity, and the initial white dwarf luminosity.
Observations of elemental abundance enrichments in these classical nova events
imply that the ejected matter consists not only of processed solar material
from the main sequence partner but also of material from the outer layers of
the underlying white dwarf. This indicates that white dwarf and accreted matter
mix prior to the thermonuclear runaway. The processes by which this mixing
occurs require further investigation to be understood. In this work, we analyze
elemental abundances ejected from hydrodynamic nova models in search of
elemental abundance ratios that are useful indicators of the total amount of
mixing. We identify the abundance ratios $\Sigma$CNO/H, Ne/H, Mg/H, Al/H, and
Si/H as useful mixing meters in ONe novae. The impact of thermonuclear reaction
rate uncertainties on the mixing meters is investigated using Monte Carlo
post-processing network calculations with temperature-density evolutions of all
mass zones computed by the hydrodynamic models. We find that the current
uncertainties in the $^{30}$P($p$,$\gamma$)$^{31}$S rate influence the Si/H
abundance ratio, but overall the mixing meters found here are robust against
nuclear physics uncertainties. A comparison of our results with observations of
ONe novae provides strong constraints for classical nova models.
