Juan Carlos
To get the best of the 30 min ... use up all 30 minutes! 
The 30 minute guideline is just that, a guideline for optimal PR'ing, studying. Any learning activity should be rewarded with a break. Make's for a more enjoyable experience because studying does not become monotonous.
To get the 'BEST' out of the 30 minutes requires you be able to commit 100% concentration.
No one has weaknesses, just abilities that need room for improvement!
There are other forms of activation besides mind probing. If your texts contain questions at the end of the chapters, perhaps you can use those as mind probing questions. Remember, the other forms of activation serve a purpose as well.
Your english is perfectly fine!
