A simple analysis of the problem of solving stellar orbit around Sgr~A* to derive absolutely (geometrically) the distance to the Galactic centre, $R_0$,.. was used to demonstrate that neglect of plausible radial velocity of Sgr~A* relative the LSR (20--60~km\;s$^{-1}$) leads to a substantial systematic error in $R_0$---of at least 1.3--5.6\%, i.e.\ ${\sim}0.1$--$0.5$~kpc, for star S2/S0-2. Sgr~A*'s tangential motion can measurably increase this error. Relativistic and non-Keplerian ${\cal O}\left(V_r^2/c^2\right)$ effects should also be allowed for in orbit modelling to avoid an additional bias of the $R_0$ estimate.