There are no rules but just from looking on other uploaded speedruns you can guess what current rules are. Stop timer right after crossing the finish line of the last race in the run seems to be current rule and runs usually begin either in main menu for 100%/arcade/championship or right after clicking championship or arcade mode. We can make clear rules but there are no mods active. I don't think we should start championship % at first track, buying is also important - choosing cars etc. and it's part of this category. I would vote to start timer when you click championship/arcade mode from main menu.

It would be better and make much more sense. There are two possibilities: complete every path in arcade mode meaning 6 times 4 tracks or every route in arcade mode so 10 times 4 tracks.