Committing key features of the low level Nav route and target runs to memory saved a huge amount of faff, and helped you stay ahead of the cockpit work cycle. It minimised the requirement to be heads in. But, if you had to, it was always do it in small chunks (known as cross check time in US parlance)
Oh, and as others have said, don't be the douche that kills others on our roads because you're playing with a phone. Activity monitors now allow police to check after accidents and modern phones can sense sudden impacts (I.e. The moment of the crash).