The article mentions optimum spacing if you have an exam coming up. Has anyone found useful references in regards to optimum spacing if you want to permanently memorize information?
The schedule mentioned for exams is not used in most SRS; they already target permanent memorization (which is one reason why they sometimes need a 'cramming mode' for students who are willing to pay extra reviews in order to have cards memorized by a particular date).
You kinda have to find what works for you, but a good rule of thumb is to double the time - e.g. review after 1 day, then 2 days, then 4, then 8, 16, etc. If you get a card wrong, it goes back to 1 day. Pretty soon, the cards you know will be permanently memorized, and should only get reviewed in 6 months or a year, etc.
The rule of thumb is that retention interval correlates directly to spacing gap (ie., the longer you wait to practice, the longer you retain). That's one reason that SRS systems use expanding intervals.