Sorry, ignore the part about setting V. Refresh. That’s automatically detected by the OSSC.
In line1x mode with a V. Active of 322, there’s (525-322=) 203 lines offscreen.
In line2x mode with a V. Active of 424, there’s ((525-424)*2=) 202 lines offscreen. Because this is line2x, we can’t test with exactly 203 lines offscreen.
It sounds like your TV supports a maximum of 203 lines offscreen.
Building on that, in line1x mode if we can’t go below 322, then the aspect-ratio-preserving setting you should use for H. Active is (720/480=x/322; x=) 483, which is a resolution of 483×322. For line2x, we can’t go below 424, so that puts the H. Active at (720/480=x/424; x=) 636, which is (636*2×424*2=) 1272×848. Of those two (line1x and line2x), 1272×848 is ((1272*848)/(483*322)=) 6.9x more pixels than 483×322, while the actual image is only 4x more pixels (2x in both dimensions), meaning that line1x should produce the most “zoomed in” picture, but it’s probably going to be less sharp than line2x because your TV is doing more of the upscaling.
Also, neither of these resolutions divide well into 3840×2160 (4k). So let’s figure out what the nearest resolutions are that let the TV integer multiply up to fit the screen.
For line1x, the next nearest integer above 322 which when multiplied by a whole integer produces 2160 is (2160/6=) 360, and preserving the aspect ratio that’s (3840/6=) 640. That’s a 640×360 resolution, and you can get that by setting H. Active to 640 and V. Active to 360.
For line2x, the next nearest integer above (424*2=) 848 which when multiplied by a whole integer produces 2160 is (2160/2=) 1080, and preserving the aspect ratio that’s (3840/2=) 1920. That’s a 1920×1080 resolution (aka 1080p). Unfortunately, line2x does not produce enough total lines or pixels to get to 1080p, neither horizontally nor vertically. Including all of the offscreen signaling (some of which is actually needed), line2x is 1716×1050. So an integer division of 4k in line2x mode isn’t possible for you.
So your options are:
1. Most zoomed in – 483×322
2. Most sharp(?) – 1272×848
3. Even multiplier to get to 4k (might be more sharp than option 2) – 640×360