The automated calculations are a blessing but also a curse. Although we greatly benefit from the fast and accurate results provided by automated computation, the lack of practice of mental calculations 'atrophies' the brain much like a muscle lacking exercise.
Flying (still) involves performing a considerable amount of calculations and pilots should be able to show good mental math capabilities. So, keep practicing to keep sharp.
The mobile application allows you to practice, to check, and to enhance your mental maths skills. Have fun!