As seaham said a
refractometer ($39.99 on sale) is your best bet for dead on accuracy. Floating hydrometers and swing arm meters are fine for FO but are prone to giving less the accurate readings due too improper calibration or physical damage.
If you want to reduce your sg because it’s too high or low I wouldn’t do it more then .002 per 12 hours i.e. 1.025 to 1.023 or 1.023 to 1.025.
Easiest way is to remove 5% SW and replace with FW every 12 hours to reduce or top off with SW each day instead of FW to increase till you reach the appropriate levels.
IMO ph/sg/temp matching is most critical if you have inverts (starfish being most sensitive). If fish only then minor variations no greater then .002 in sg, .3 in ph, or 3 degrees in temp will not have an adverse affect.
Variations greater then those can cause osmotic shock and impair some stock to the point of death which is why it’s important to not only mix properly with a ph/pump for 24+ hours but also to try to match as closely as possible.
The size of the pwc is important to consider when matching as well since being slightly off when doing a 10% pwc is not going to affect the overall parameters as much as doing a 50% pwc for example. That is also why most advocate doing smaller weekly pwc versus large monthly pwc to reduce the risk of shocking your stock.
When mixing most artificial salt they advise ½ a cup per gal which usually gives you a sg around 1.022. Follow the directions and after 2 hours check your sg and adjust accordingly. If you consistently mix the same amount each time you will learn the exact amount of salt within .002 you need to add without having to make any radical adjustments.