From [url=https://raw.githubusercontent.com/SeeedDocument/Grove-CO2_Sensor/master/res/MH-Z16_CO2_datasheet_EN.pdf[/url] the data sheet:
- Notes for maintenance
7.1 The sensor should be calibrated regularly. The cycle time is better to be no more than 6 months.
I think this is particularly relevant as you're on the bottom end of its range. Also it does have a stated error of +/-50ppm.