A few recent cameras have a limited amount of memory available, especially as CHDK has increased in size.
In addition, not all of the bitmap buffer is used for screen display.
In the case of the SX200, an area 720x240 at top-left of the 960x270 buffer is used.
So, we only need to allocate memory for 960x240.
That is 30 rows less so ZEBRA_HMARGIN0 is set to 30.
You could save even more space by reducing width but that is more complicated because it would not be a continuous area of memory.
The 960x240 'letter-box' is a continuous area of memory that can be quickly copied using a single instruction.
I am not familiar with CHDK menus but I know you can display amount of memory available.
With zebra disabled, how much free memory is there ?