  Absorption features in stellar atmospheres are often used to calibrate
photocentric velocities for kinematic analysis of further spectral lines. The
Li feature at $\sim$ 6708 {\AA} is commonly used, especially in the case of
young stellar objects for which it is one of the strongest absorption lines.
However, this is a complex line comprising two isotope fine-structure doublets.
We empirically measure the wavelength of this Li feature in a sample of young
stars from the PENELLOPE/VLT programme (using X-Shooter, UVES and ESPRESSO
data) as well as HARPS data. For 51 targets, we fit 314 individual spectra
using the STAR-MELT package, resulting in 241 accurately fitted Li features,
given the automated goodness-of-fit threshold. We find the mean air wavelength
to be 6707.856 {\AA}, with a standard error of 0.002 {\AA} (0.09 km/s) and a
weighted standard deviation of 0.026 {\AA} (1.16 km/s). The observed spread in
measured positions spans 0.145 {\AA}, or 6.5 km/s, which is up to a factor of
six higher than typically reported velocity errors for high-resolution studies.
We also find a correlation between the effective temperature of the star and
the wavelength of the central absorption. We discuss how exclusively using this
Li feature as a reference for photocentric velocity in young stars could
potentially be introducing a systematic positive offset in wavelength to
measurements of further spectral lines. If outflow tracing forbidden lines,
such as [O i] 6300 {\AA}, are actually more blueshifted than previously
thought, this then favours a disk wind as the origin for such emission in young
stars.
