Unfortunately, it wasn't as simple as that. While the manual describes the feature, it is out of date. To prevent accidental changing of the offset, a special power-off/power-on sequence has to be followed before the setting becomes active. Polyscience provided a bilingual (English/French) loose-page addendum to the manual describing the necessary steps. I followed them, but I still couldn't set offset.
After trying every possible interpretation of the described steps, I mailed Polyscience. They quickly replied and after some back-and-forth, it was determined that an error had crept into the addendum: instead of the power button, you have to press the menu button while connecting the unit to power. Here is the corrected sequence:
- Disconnect power cord
- Plug power cord back in while pressing the menu button
- Press menu button repeatedly until "offset calibration" is visible
- Change offset using up and down buttons
- Press menu button again or wait 10 seconds until the device shows the current and set temperatures again