You might want to try this...
Put a medical thermometer in a small lukewarm glass of water.
Place in incubator and let sit for 12-24 hours.
Lift out the glass of water and check the temperature on the medical thermometer...take the reading with the thermometer still in the water.
Compare to digital thermometer.
Make mental sticky note of the difference and "adjust" the digital thermometer reading accordingly.
For example....If medical thermometer reads 98.3F and your digital is reading 95.9F then you know your need to add 2.4F to the digital reading. If the med thermometer reads 98.3F and your digital is reading 99.9 then you know you need to subtract 1.6F for the digital reading.
Just some thoughts...
Ed