You got it, though you'll never reach zero. The exact numbers for tier promotion we keep under wraps so people don't attempt to walk the line and sandbag a lower tier. The weighting we use has an impact on your average as well since your more recent scores have a stronger influence in the calculation.
The mark to hit pro is around 72-73 somewhere I believe, but I recommend just playing to your potential. As you play more rounds your scoring average will improve (or not) and if you post all of your rounds you'll gradually work through the tiers at your pace. If you do that you are certain to be competing in a tier with players that match your skill level.