I've read from many people that they start their balls on rat pinks from day one. I would guess that it's never too soon, so long as you can get the appropriately sized rats you need. But some people may have stronger opinions than I do on this issue.

As for that primer method...it SOUNDS good. Good luck trying that with a snake who is too shy to eat if you're in the same room or whatever. Meaning...I'm sure this method works often...but it can't be guaranteed to work with every snake.