Rule of thumb,that can be derived from the ICC calculation methodology is this:
If a team wins 50% of its matches and loses 50% of its matches over a period of three years it will have a rating of 100.
If a team wins 25% of its matches and loses 75% of its matches it will have a rating of 50.
We can see from the table that all major teams have a rating of 100 and above. So another rule of thumb that can be derived is : a major team is one which has a 50-50 chance of winning a match that it participates in over a period of 3 years.