Pair-up efficiency as a function of the central SMBH mass. The vertical axis shows the fraction of BHs that successfully form a binary during the simulation, while the horizontal axis shows the logarithm of the SMBH mass. As in Figure 3, different marker fillings indicate the BH generation (filled: 1g; hollow: Ng), color encodes the viscosity parameter α (orange: 0.01; teal: 0.1; navy blue: 0.4), and the two torque prescriptions are distinguished by marker shape and line style (squares and solid lines: G24; circles and dashed lines: B16).