10# honey/3.5 gal of water should yield a sg of around 1.092-1.100 off the top of my head. I don't trust the refractometer calcs on FG, so take a hydrometer reading. Your high finishing gravity could be from low pitch rate, fermenting too low for yeast strain, lack of nutrients. Champagne yeast is noted for stripping out the must. Start with the hydrometer, then report back.