A cylinder style is not a torus and will give bad results with focus at the edges no mattter what and the middle and edges will have odd hotspotting compared to the flat top and bottom if you use gain. Plus either you'll get the flat part to focus or the curved edges but not both. Imagine a windows taskbar 6" or whatever out of focus at the ends or in the middle. Yuk!
The torus curve gives no gain by itself btw. In other words white torus is pointless unless you just want to create a flight simulator cockpit window effect. The purpose of the curves is to remedy falloff of gain on the edges of high gain materials and focus the gain to you so that the whole screen becomes one big even hotspot.
To calculate the optimum curve take a piece of string and fix it where your head would be when sitting in your premium centered seating area. Estimate where you want the center of the screen to be depth wise and move the string to where the edges would be and measure the difference of depth all around. (This is very simplified and just gets you a rough idea.) Pj mount position determines the angle the screen would reflect back to the seat (like a mirror) so you would want to figure that angle the screen would be tilted too, if any, somehow. Plus it's likely you'll want the focus two or three seats wide rather than just for one seat so then give it a wider curve. If you aren't using a set of lenses specified to handle a full torus then a torus lite with less curve is better than the full curve so you can still maintain decent focus around the edges. Half the curve or so for lite, which also gives a wider optimum seating area before falloff.
You get the basic idea right?
Play away, I know nobody is listening and we all tend to do what we want to do no matter if someone warns us it's wrong or wont work well anyway. I'm just telling you what to expect.