We perform the scale setting procedure of a mixed action setup consisting of valence Wilson
twisted mass fermions at maximal twist on CLS ensembles with $N_f=2+1$ flavours of 𝑂(𝑎)-
improved Wilson sea quarks. We determine the gradient flow scale $t_0$ using pion and kaon
masses and decay constants in the isospin symmetric limit of QCD as external physical input. We
employ model variation techniques to probe the systematic uncertainties in the extraction of the
ground state signal of lattice observables, as well as for the continuum-chiral extrapolations used
to determine $t_0$ at the physical point.