Any test needs a baseline.
It makes sense to place a DE or Feather blade
in a cork and keep it handy to calibrate
the hair in question.

Humidity, cleanliness, direction and normal
variability all interact so keeping a standard blade
may prove the best way to calibrate such
a test.

For me the HHT is best used to compare toe
to heal on one edge not razor to razor. A single
hair from my SO's hair brush slides on dull, and
grabs on sharp. Her hair is dark enough that
I can see it... seeing is a big plus as is the length.