I agree, just turn it up, but do it gradually, and use a second thermometer to double check the calibration.
I have a Visi-therm 100 in a 29 gal FW
, and it frequently is off by about 3 degrees. I've set it at 78, but digital thermometer readings consistantly show 81 degrees.
I had a Visi-therm 200 in my 30 gal FW
and was generally off by 1 degree. My new Ebo Jager has so far been accurate to +/- .5 degree.