I used a digital conductivity/salinity meter to read concentration, so I can't help with a refractometer.
I would put a handful of salt in a HOB filter, another handful in the sump, and let them dissolve (a few moments), and add another couple handfuls, until the meter read at least 3ppt. In this way, over a few hours the fish got used to the salinity.
Even though only 2 of your fish show signs of ick, you should assume they have all been exposed, every time a white spot disappears, it means the spot has exploded into up to 100 new ick parasites, and when the new ones hatch out, this is when the salinity kills them.
By raising the temp, the ick spots mature faster, and this warmth hastens the ick life cycle, also allowing treatment to speed up and kill the emerging parasites faster.
The treatment does not work over night, it can take weeks. So everything you do a water changes, you will need to add more salt to keep the concentration up.
And doing water changes with vacuuming helps, because there are always some inert cysts in the substrate (eggs, for lack of a better word) waiting to hatch out and glom onto the fish.