Yes Steve,

on such ideal and non practical conditions you are right.

On working with your accu drill you can see a big
variation of the speed depending from the load.

You can never calculate the absolute count
of turns of a DC motor only with the input voltage
and the power on time.
You need always a sensor to do it


A special way was to use a DSP to measure the input current
over the time. Than you can detect a short current changing
if the commutator switch to the next coil and can count
this. So 3 current changings on a motor with 3 coils are
equal to 1 turn.

There are some specialized DSP made to control DC motors like
stepper motors. But I can not remember, which company.


Peter