IMHO the leaderboard is still empty. The starting point will be easy to decide based on the first submited run.

I have ¤so far¤ chosen to start at the first race as it lets you get difficulty options and suchlike set, just because I hate driving with some of the assists in the default options. I do agree it'd be more streamlined to start at DiRT TOUR though.

Fyi you can set your preferred difficulty and assists before starting the run by simply doing a quick race and setting them from there as it carries over into career mode.