Honestly, I has the same question too wen I started a couple of months back. Had no idea how much salt to add and worse, didn't even know how to operate the hydrometer lol.
Well, it's all about agar-ration initially. Then let the salt dissolve and test with a Refractometer. Would recommend Refractometer cause hydrometers are usually more inaccurate. Once you had read the initial salinity, add either more salt or water to achieve the salinity you need/want.
Once you do it a couple of times, you will get better at it. Practice makes perfect!!