Well, I finally managed to get the text to fill the top and bottom of the screen using ALL of these commands:
--headerstyle=none --headerheight=0 --topmargin=0 --sidemargin=0 --screenheight=800 --screenwidth=650
The only problem is, I still have the thin blank spaces on the left and right sides. I started with a screenwidth of 600 and worked up to 650 in increments of 10, but there were no changes visible in any of my tests. I've given up figuring this last part out, unless someone has an inspired command for me to try out (I even tried using negative values for --sidemargin, but I got an error from the program).