Not sure about the conversion ratio, I think that an sp of 1.025 = 35 ppt salinity. See if you can locate a reflectometer--they will help out. Check Instant Oceans webpage, too, as I think they have a chart.
All salt mixes contain buffers to raise the pH of water to sea-like levels. A sudden change can be stressful to fish, but the amount of buffer needed for 5ppt salinity should not significantly change the pH of 440 gallons of water.
Dechlorinating is very important. While the levels in most water are not high enough to damage the bio-filter, it can be irritating for the fish. Aging works for chlorine, but some places treat with chloramines, and these must be treated (along with the resulting ammonia--Amquel works well).
Salt mixes are fairly pricey. Buying larger quantities drops the price some--I would look for the 5 gallon buckets. Seldom does salt go on sale--though online stores offer it for cheaper, until you pay shipping. Several places will do sales where shipping is free, and this is a real bargain. Just gotta' watch for them.
For adding salt--you only add salt to the water used for water changes. Salt should be dissolved into this water before you dump it into the tank--you do not want to add salt directly to the tank. Evaporation removes water only, leaving salt behind, so you will want to topoff the tank with freshwater only. Otherwise, your salinity will spike rapidly.