@aart , has a great article on how to calibrate thermometers, you can do a search on here for the article and  learn how to calibrate thermometers. It's a matter of getting some water to 100°F or a little above, then seeing what the thermometer reads that temp at...the reverse is cold water, 32°F and see what the thermometer reads. The difference is what you will use to know the correct temp. It's a simple % difference equation.
.3°F is not going to 'make' or 'break' a hatch! Never, ever trust the manufacturers instrument (incubator)  sensors, they are notoriously inaccurate and unreliable.