You still didn't mention anything about your hydrometer.
That question can't be answered without knowing how your hydrometer was calibrated.
If you use the plastic swing arm, it won't make a great deal of difference as they seem to work well at quite a large range, at least for the range normally used in the hobby. However they are not known for their accuracy right from the start so they need to be calibrated against a known calibrated standard first. They also need to be rinsed after each use, and, cleaned approx. weekly in white vinegar overnight.
If it is a glass floating hydrometer, you need to know what temperature it was calibrated at so appropriate adjustments can be added. If you look at the paper inside the tube, there should be information to this effect printed on that paper.
If it is not a certified hydrometer, again, it should be checked against a known calibrated standard before depending on it.
If it is a hydrometer sold specifically for the hobby, it is generally calibrated for temperatures normally found in the reef hobby, but again, the accuracy can be suspect until compared to a standard.
Also, for accuracy, the glass floating hydrometers should not be used in the aquarium, but the water should be transfered to a cyclinder where there is no water movement to create a false reading on the hydrometer.
I clean mine on a regular basis, again, with white vinegar, to keep proper calibration.
