Coaxing a completely black background out of my portraits

I am going to be doing some tests/portraits with T400CN, an Elan IIe, 50 and 85 f/1.8, 2 550EX wireless via the ST-E2 transmitter on light stands. I would like the dark colored background to all black but *just* black not past it. Should I externally meter the ambient and underexpose 2 or 3 stops? How bout choosing my aperture and shooting at 1/125th in manual mode?

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