Relative humidity (RH) is measured as a
percentage--absolute and specific humidity are NOT.
If it reads in % it's a relative gauge. RH gauges are
fairly inexpensive and commonly available. Gauges
measuring absolute or specific humidity are hard to
find and expensive; they also read in either g/kg or
g/cubic meter (or the non-metric equivalent) NOT %.
"Actual humidity" and "exact humidity" are not
measurements.

Dial RH gauges are rarely calibrated properly by the
time they reach your home. This is NOT the fault of
the manufacturer. It is simply the way it is with
this type of hygrometer whether it's for your tank or
for your house. This probably accounts for the wide
variation of humidity levels that have been
recommended. You can do something about this though
:)

Calibrating your hygrometer:
First check to see if your hygrometer can be adjusted.
Look at the back to see if the small center post has
a slot for a small screwdriver. If it does you'll
need to have a screwdriver ready to make any
adjustments as soon as you open the jar. If it
doesn't, don't worry, this procedure will still allow
you to use your hygrometer more effectively.

Half-fill a shot-glass or other small dish with table
salt and add enough water to wet the salt completely
but not dissolve it--it should be like wet sand. Put
the shot-glass and hygrometer in a wide-mouth jar, and
seal it airtight. After six hours (or more), the
humidity in the jar is 75%. The hygrometer should read
somewhere around 75%. If it does not, take the
screwdriver and turn the screw on the back of the
hygrometer until the meter reads 75%.

If your hygrometer is non-adjustable and reads other
than 75% in the jar, you must correct all subsequent
readings by this difference. Example: if the
hygrometer reads 67% in the jar, it is reading 8% low.
Thus, it will always read 8% less than the actual RH.
Using this example if you wanted the humidity to be
60-70% then the reading on your hygrometer would have
to be 52-62%.

Since 75% is right in the middle of the recommended
range for hermit crabs I just mark my non-adjustable
gauges with a black marker where it points after
calibrating. Then I know it's in the right range as
long as the pointer is on, or slightly to either side
of that mark.

Dial RH gauges can be off by 20% or more! They are
also usually only accurate to +/- 5% or worse and
often take up to 2 hours to finish stabilizing to a
new reading if the humidity changes. Please remember
that it is simply the nature of the beast and NOT
necessarily poor quality or manufacturing. The spring
mechanism is delicate and goes through a lot
(shipping, etc) before reaching your home.

If you want to be able to measure humidity more
accurately your best bet is a digital hygrometer.
But, not all digitals are more accurate than dial
gauges--you need to look for one that is accurate to
+/- 1-2%.
I hope this helps clear up some of the confusion
regarding humidity and hygrometers.