Thanks for posting up the pictures Nephasth! I did not do that much testing at 1024x768, so I'm glad to hear you liked the results with a vertically orientated monitor.
When you get a chance, check out the mini-SLG for your 60-in-1 board. One thing I tried to do with my design was to not use straight black scanlines. I felt that on the low-res games (Pac-man, etc..) the detail suffered and felt even lower res. So I kept the scanlines at around 15% intensity and found that the scanline impression was still quite nice, while not degrading the game resolution. Let me know what you think!
As for the pot adjustment, it controls the intensity of scanlines for the Red channel, versus the Blue channel. The effect will be more pronounced on low-res games, at 640x480. I've actually been thinking about dropping the pot from the design. As you discovered, it is hard to judge unless you are looking right at the monitor, and the effect is more pronounced on low-res games, and not so much at higher resolutions. Plus I do like the idea of the mini-SLG as a true plug-and-play adapter, with no switches or adjustments to worry about. Instead, I would use a fixed resistor to strike a good balance between the R,B signals.