For all categories (including GBA) I initially chose the moment you start moving your 1st car. I didn't consider that key characters are already moving around as soon as you enter the level. I changed the PC categories' start to pressing the Start key in the Car Select Menu on your first car. The GBA run starts when you confirm your control settings and enter the intro Circusville cutscene.

I'll change the time of my PC - Any% run accordingly.