Having the ability to do quick calculations in your head is critical for good engineering. I've worked as an electrical engineer and knowing a lot of 'envelope

Having the ability to do quick calculations in your head is critical for good engineering. I've worked as an electrical engineer and knowing a lot of 'envelope-style' calculations is important when you are trying to trace down problems. Being able to calculate parasitic inductance, or capacitance is key when say trying to find out why and opamp is oscillating.