Not sure if available in the US,
but there are adjustable hygrometers.
They have a screw you can use to adjust the arrow.
If you put the hygrometer in a moist towel for about 15-30 minutes,
the arrow should indicate something close to a 100%,
if not, you can use the screw to adjust the value to about the 100% since you know that should be around the correct value.