First, please answer about the desired accuracy of your measurement as in the first part of reply #22.
If you verify a resistor with an ohm meter, you've only verified the resistor. Not the measurement device.
I'll just fast forward - you should have a software calibration step "please attach reference resistor and push button" then the sketch will store and use that value. Actually, it would be a pair of resistors. You need the test resistor already shown in the diagram, and a calibration load resistor.
I suggest a rotary switch to select different test resistors (to replace the test resistor in your diagram).
Thinking further, you could calibrate it periodically (once a day, week, month, year) and store the values in EEPROM so the end user doesn't have to mess with that. They would only need to turn the rotary switch.