these are some really basic questions...are you sure you are ready for this tank? frankly, i see disaster coming....
salt doesnt evaporate, only h2o, so if you replace evap with SW you are raising the salinity. you do need a mark somewhere on the back (but visible to you) so that you know how much evap there has been and how high to refill
as far as your salt, you will just have to use your hydrometer and fiddle...most salt comes with estimates such as 1cup = 5gallons @ 1.0024