Your rule of thumb to multitask effectively should be to

Your rule of thumb to multitask effectively should be to never allow any task more time and attention than it is needed. Because if you do, you cannot perform multitasking because you are devoted to a particular task only. For example, when answering an exam, you need to read every item carefully. If you cannot solve the item you are currently reading, skip it and proceed to the next one to save time. 

Afine-grained igneous rock forms as a result of slow cooling.