Your salinity level is among one of the more important parameters in a marine aquarium. Maintaining a proper salinity level will help you establish a proper pH as well as making your alkalinity and calcium levels a lot easier to maintain stable.
So what is the proper salinity level ? Many people have many different opinions about this ranging from 1.020 to 1.029 and they will also have a thought process in support of their options mostly around the type of setup they have. As the more popular fish and corals that we keep in the hobby come from in or near the ocean’s coral reefs with a salinity of around 1.026, I prefer to keep the salinity levels my FOWLR and Reef aquariums at the same level as these areas of the ocean, 1.026 (35ppt). However, there are some exceptions to that. The Red Sea for example will have a salinity level of around 1.028 to 1.030 depending on the exact location making it better to have a slightly higher salinity level if keeping fish or corals from the Red Sea.
You really do need to understand the water parameters in the area of the ocean that your fish and corals come from before you develop your own approach to maintaining your salinity.
The below link is to a very handy article that contains instruction for very different types of marine test kits