Seems to me if you use a target solution that you know is 35 ppt and calibrate the refractometer at whatever temperature you usually use the refractometer (assuming it is auto-temp correcting and you give it time to allow the temp to equilibrate as Jack said), it shouldn't matter at all what the temperature is since both your solution and tank water will be measured by the same instrument at similar temperature. Unless, of course, there is something temperature dependent about the way the solution is made.
However, even if the temp is throwing you off a bit, the error is only going to be 1 or maybe 2 ppt on the calibration. I think fretting about that small of a difference is like peeing into the wind...
Stability and getting into the right ball park across time seem to be the bigger issues. If you auto-topoff, the salinity in the tank should stay pretty stable and even if you're a bit off for a water change, a 10% water change that is only a couple ppt different from the tank water isn't going to change the overall salinity of the tank much at all. There are plenty of other things to worry about that are going to do a lot more harm to your tank than being 2 ppt off.