A couple points is not a big issue. If you want to be anal, just right on them +2% or whatever. You could also try the moist salt test (should read 75%) to give you two points of calibration.
- Place the salt in a bottle cap (or another small container).
- Dampen the salt with water. Not soupy. Pretty much like a dry paste.
- Place both the hygrometer and the bottle cap full of damp salt in a Ziploc bag and seal to seal out the air in it while the test is going on.
- Go to sleep.
Keep it like this for a day or so then see what it says. It should read 75% if the ziplock environment is perfect.
As for temps I don't worry about them too much. My place is in a controlled environment when it comes to temperature. Again a few degrees here or there + or - is not a big issue.
That's my 2 cents on the topic.