Aspect Ratio info;
From measurement the OSSC in Generic x3,x4 modes creates aspect ratio of 4:3 according to 224 vertical lines, so 224*4/3 = 298 horizontal.
That means 22 pixels horizontally and 16 pixel vertically are overscan.
Also, games that use 240 vertical pixels on the Genesis or Snes will have 300/240 (3.75:3) aspect ratio.
My CRT is set to show the Genesis and Snes halfway between 224 and 240, so 224 is slightly underscan and 240 is slightly overscan, for best of both worlds and support for PS1 (240) without overscanning too much.
This is an image of Genesis 240p suite in Linex3 Generic mode including the borders.
Masking tape marks are 4:3 ratio on this 16:9 monitor.
Now a PS1 game is 320×240 but I can see that horizontally it is not fully 4:3.
Looks like OSSC Generic mode treats 320 horizontal lines as active+overscan.