Hi Chazz
Yes, the thread you found answers your question. I will put a link here to make it easier for everyone else to find:
New Design Balancing system when changing leagues
What Jack says in the very first post is still correct. So the best time to join a new league is during the break between one season and the next. If you join at this point your car will be competitive because your dp allocation for Australia will be as if you raced in all seventeen rounds of the previous season with your "active" CD at the time you joined the league.
If you join at any other time you will not be competitive. If you join early in the season it gives you time to get your car up to speed for the last few races and avoid relegation. If you join a competitive league late in the season with enough teams to activate promotion/relegation you are almost certain to go down.
Race 13 is risky but seeing as there are only 12 teams in the Pro tier of the league you just joined you should be safe. If you find your design after Abu Dhabi is too low because you only competed in four races you can reset it by leaving and immediately rejoining the league.