When I’m using salt I’m not as familiar with I first add about 80-90% of what I think I’ll need, fully mix it, then test salinity. Then add more as needed to get to target salinity. Keep track so next time you can just add the right amount.
Also when you say just dump the salt in, just wanted to point out that you shouldn’t just dump a whole bag of salt in. You need to add it in smaller amounts while mixing, letting each addition mostly dissolve before the next. If you just dump it all in you are more likely to have problems with incomplete dissolution and precipitation, that you won’t be able to fix with just more mixing. I made this mistake routinely during my early years of reefing.