NSW is 1.0264 @ 35 ppt.
I have no clue why a refract would have 1.025. What the are probably looking at is corrected Density, which is about one unit lower, ie, 1.025 instead of 1.0264. Density does not equal Specific Gravity.
That RS stuff is just showing what levels you will get at different mixing rates with their salt. They do not show one for 1.0264, for oblivious reasons. Meaning, the levels of Calcium would be ~ 500 ppm. NSW Calcium is 420 ppm Calcium @ 35 ppt and the have 420 @ 1.021, 30.6 ppt. You are reading into the RS stuff, as if their last mix is 1.025, same as your refract or the Sper.
Sper Scientific
Where did you get that 1.025 =35 ppt. I do not see it on their website. They only have one Hand Held, NaCl refract. I see no image of the lens. And Sper actually does not manufacture allot of their stuff.
http://www.sperdirect.com/salt-refractometer-w-atc-0-229-prd1.htm
Vee Gee, on the filter guy, is a real refract manufacture and the real name of the company, that actually makes what they sell.
http://www.veegee.com/index.html
Look at this table
Temperature Corrections for Hydrometers
http://reefkeeping.com/issues/2004-07/rhf/index.htm
Now, can you have 1.025 = 35 ppt. Yes, look at the table, @ 29 C ( 84 F ) it is 35 ppt. As I said your refract and the Sper are probably incorrectly using the term Specific Gravity for D.
Calculator
http://reefkeeping.com/issues/2004-07/rhf/conversion.htm