(I researched how to turn gray background into black or white or any other color--I'm excited to try this!)
I don't remember seeing anything about you having a light meter that reads both Reflective and Incident. You can set your lights by guess but, it's a whole lot easier and faster with a meter.
Turning an 18% gray background black or white involves adjusting light on the background so the "reflected" meter reading goes up or down. It takes 4 stops either way. If you're interested I compiled a chart from the Dean Collins Chromozones method. You can down load a PDF copy of it here
Dean Collins Chromo Chart.pdf
For example using the center grayed column if your "reflected" reading from the background reads f/11 for 18% gray, then adjust your background light to get a "reflected" reading of f/16 for a 36% gray background or a "reflected" reading of f/8 for a 9% gray background.
Where this becomes a problem is if you have ambient light or spill from other lights on the background. That's part of the reason for the separation between the subject and the background.