A refractometer doesn't measure specific gravity, just the the sucrose of the wort, or wort gravity. In order to make the tool do the calculation to convert to SG you have to calibrate every wort, since there are more sugars present than just sucrose in any given batch. To calibrate the wort, you need a hydrometer reading to get the brix correction factor or else your calculation will be off. Not only do you need to calibrate the refractometer you have to calibrate the wort. A simple and accurate way to check the gravities is with a hydrometer. Maybe its just me... I like things uncomplicated!
Good luck with your correction calculation!
Just enter the ingredients section, yeast, 1968, and edit the max and minimum attenuation until you get the number your looking for.
Just wondering if the yeast is from the same date or across the spectrum. If the later, it is great quality control.