You don't have to be an engineer to caclulate steps for Mach3. Its just a series of ratios from the stepper at 200 steps per rev, the drive which is 16 microsteps per step, the pulleys are ratioed just like gears in a gear train. you also use C = Pi * D to convert the rev is steps per rev to a linear distance. Just check what ever you come up with by confirming that it jogs the correct distance.
For speeds and acceleration I just use what ever seems smooth. On a printer you can tell when its too fast because the print will shift.
In my opinion I would configure the machine in metric. All prints that you might download on line are in metric. Its a metric world, force yourself to design in metric.