The 83% I found to be best looking is for yoko in 720p only. For any other resolution just check the various settings (just press the back button during play, brings up the video options). It's basically just a mathematical equasion. On a 720p output the 100% area is 540x720 pixels in size. 83% of that are 448x598. This elimates any scaling artefacts on the horizontal axis (since 2x224 = 448). If you want to eyeball the result, take a look at the grid on the bottom of the screen (where the bombs are shown), you'll notice that the vertical grid strips are uniform on a certain scaling percentage, while they vary in width on all other settings.
If playing on a 480p output in Tate 93% (= 448 pixels active image width) should look best, but you get a bit of underscan border around the actual playing area. Bicubic and Bilinear filtering eliminates all the scaling artefacts. Unfortunately 5pb didn't properly implement the N.Neigbhour and Lanczos filtering - this could have looked better...
Without trying myself I would recommend 480i/480p with 93% on a CRT. On an Emotia/XRGB setup I would try 100% along with Bicubic sizing, possibly pumping up the sharpness on the XRGB-3 a bit!
Nevertheless, I'm amazed how good Ketsui actually is. The first three levels are a blast to play and I'm slowly getting used to the 4th level as well