Identify the vernier scale and main scale on the spectrometer
Count the total number of vernier divisions
Measure the value of one main scale division
Use the formula: Least count = value of one main scale division / number of vernier divisions
If the spectrometer has a circular scale, use: Least count = smallest division on the main scale / total vernier divisions
Check the instrument manual or calibration label if available
Verify the least count by comparing adjacent vernier and main scale markings
Record the least count in the same units as the scale