Did some science here using both BizHawk (with Genplus-gx core) frame counter and VODs from submitted runs so far. It takes 158 frames (2.63 seconds) from pressing Start Button to the moment your character can start moving.

So instead of starting the timer from the moment your character starts moving (aka SDA timing), start your timer when pressing 'Start' on Main Menu. Set your split program to start from -2.63 seconds, or subtract this from final split.