Calculate RAID-1 (mirroring) usable capacity: Total = drive_size (mirrored pair uses only 50% of raw capacity). Survives failure of all but one drive.
Enter the values for the scenario you are assessing. Calculate RAID-1 (mirroring) usable capacity: Total = drive_size (mirrored pair uses only 50% of raw capacity). Survives failure of all but one drive. Use the raid-1 capacity result to inform your calculation.