Do you have the new style Genesis 1588 with the digital display at the top? If so, it should be fairly close to the correct temperature.
Set it for 99.9 and leave it for a day to see what it says at egg level. I would ignore the mercury thermometer and go by the Brinsea Spot Check.
It has been going for a week or better! With the spot check in the center about the top of egg level the temp has been 99.8-100 degrees! It is the new 1588 but the last time I hatched in it the temp spiked to 102 about 3 days before hatch! So I moved the bator temp bown. When I pluged it back in to get it ready to hatch the temp was low. Do you get high readings when you test on the sides?