Here you are the rough guide I wrote myself, that I follow when I have to deal with analog captures - calculated with square pixels in mind, 640x480 NTSC full frame, 768x576 PAL full frame; captured at 720 pixel width, as most capture cards' native horizontal resolution have this width:

AR DAR NTSC frame mod8 (16) 1.33 4/3 720 x 480 1.66 4/3 720 x 386 (384) 1.78 4/3 720 x 360 (352) 1.85 4/3 720 x 344 (336) 2.20 4/3 720 x 288 2.35 4/3 720 x 272 AR DAR PAL frame mod8 (16) 1.33 4/3 720 x 576 1.66 4/3 720 x 462 (464) 1.78 4/3 720 x 432 1.85 4/3 720 x 416 2.20 4/3 720 x 344 (336) 2.35 4/3 720 x 324 (320)