Hi, marciof14.
Since it sounds like your ultimate goal is to be able to use the motor current measurements to tell when there is extra load (to get a constant torque or speed), it seems like the built-in current sensor on the high-power motor driver might be fine, and you might not need to do any special calibration like Jim posted about.
In a previous post you said with your motor attached that you read 190mA at your supply and 37.5mV on the CS pin. Taking into account the 34.5mV offset you measured earlier, that means the driver was reporting 150mA of current. That seems pretty accurate considering those are pretty low currents and that the power supply is providing power for at least the driver logic as well.
I suspect in that case the output from your code was negative because the current (and therefore CS pin voltage) was so low. The Arduino analog read feature can only read voltages in increments of 4.9mV, and 37.5/4.9 = 7.65, which might be truncated to just 7 ticks out of 1023. That means your code would output (5/1023)*(7) - 0.0345 = 0.0342 - 0.0345 = -0.0003A. You could check for a situation like that by printing the actual analog reading you get before you do anything with it in your code.
That kind of situation is why Amanda has asked a few times for you to try adding a load to increase the current to something closer to 1A. Could you try doing those things and see what output you get from your code and how it compares to the readings from your meter?
-Claire