The optimal sample rate depends on whether each specific game runs at 640×480 or 720×480. In the former case, the optimal setting is 780 samples per line, and in the latter it’s the standard 858 samples per line (the picture is ever so slightly wider than 4:3 in that case).
If you’re using Line2x mode with 480p and have Allow upsample2x enabled, the difference between those two sample rates gets less pronounced. Finding out whether a specific game runs at 640×480 or 720×480 requires some pixel peeping or detective work.
Here are the timings for square pixel 640×480 on the Xbox:
H. samplerate: 780
H. synclen: 58
H. backporch: 59
H. active: 640
V. synclen: 6
V. backporch: 32
V. active: 240
Take note if the picture appears shifted vertically. If so, reduce V. backporch to 31 or 30.