According to this 28 would give you about 10% kill on regular pears post bloom. Howver (being nitpicky here), how was that 28 measured? At your site? and how close to the trees? Could the microclimate around the pears vary Plus the temperature can vary with the distance from the ground.
Plus I would agree that the temperature guide is not an absolute. There are a huge number of variables in an actual orchard that can not be accounted for in a straightforward scientific test:
Things that I would think would affect susceptability to damage:
Health of trees
Humidity
Prior temperature to freeze
Wind
plus probably others
Chris Mauchline
SE PA, zone 6
40° 5' N 75° 51'W
~650 ft elevation