Refractometer calibration and operation

[Cider Hill Maple Farm]

If anyone that uses a REF 105 refractometer could give me some advice it would be greatly appreciated! I have followed the vague instructions on calibrating the instrument, I understand the steps, but maybe I'm to blind to see the color difference when adjusting the null, I can't see a color change when adding the distilled water to the prism. when I place the dioptric oil and the reference block on it, I see the distinct line and adjust to 78.8%, IS THIS GOING TO BE THE CORRECT READING WHEN I MEASURE SYRUP????? When I checked what I drew off tonight, it read 64% on the refractometer, and my hydrometer was bouncing off 66 -67%. I will finish this first draw off to hit the 66% mark, and if this is correct I just made my first "FANCY" ever!

Thanks In advance,

Shawn