i totally agree, in normal ops, you let the automatics do this math.
Actually, in normal ops (and before the days of EFIS screens, computers and "modern" pilots who neither know nor care about the theory of high altitude flight
) we let the circular slide rule do the math.
This is a CR-4, I have always (and still do) carried a CR-2. This one is set for a Mach of 0.45, with the CAS/PA window (the longest window) displaying the Airspeed/Pressure altitude relationship for this Mach Number.