Values of Π_ℓ=1=Π₁ for the 3D MHD merger product at X_c=0.50 (cross marker, ${\mathrm{\Pi }}_1^{\text{merger}}=20209$ s) and genuine single stars (dot markers) in a typical observational uncertainty box of log(L_★/L_⊙)±0.1 dex and T_eff±1000 K. The grey lines are MS HRD tracks for genuine single stars with masses between 15.4 M_⊙ (bottom right) and 20.0 M_⊙ (top left). The boundaries of the colourbar are set by ${\mathrm{\Pi }}_1^{\text{merger}}\pm K{\sigma }_{\mathrm{\Delta }P}$, with K=1, 2, 3.