While current tokamak experiments are beginning to utilise real-time feedback control systems to manage the plasma exhaust, future tokamaks still require validation of theoretical models used to predict the threshold impurity concentration required to facilitate stationary divertor operation. This work exploits new spectroscopic measurements of the divertor nitrogen concentration, c_N, in N₂ seeded H-mode ASDEX Upgrade and JET plasmas to
test the parameter dependencies of the power flowing to the outer divertor, P_div,outer, the separatrix density, n_e,sep, the plasma current, I_P, and the minor radius, a_min. An experimental scaling law to predict the threshold c_N required for detachment which best fits the experimental data from both devices is c_N ∝ P_div,outer^1.06±0.28n_e,sep^-2.66±0.24I_P^0.94±0.34a_min^-1.86±0.25. The dependency of P_div,outer and n_e,sep is demonstrated over at least a factor of two change in both parameters and indicates a moderately stronger dependence on n_e,sep in comparison to a 2P model. A first assessment of the machine size scaling highlights the need for similar measurements from other tokamaks to validate the results.