Awesome. Thanks for the link. I'll definitely be incorporating that into my spreadsheet.A couple things:
The refractometers that have SG scales (at least all the ones I'm aware of) aren't correct. The manufacturer(s?) used the "multiply by four" rule instead of actually putting in a conversion.
They're calibrated for sucrose solutions, but wort is primarily maltose. Typically you convert by dividing by 1.04 - so a reading of 20°Bx means the SG is actually about 19.2°Bx (1.079).
After fermentation starts, the alcohol in solution will result in an artificially high SG reading. You can try to compensate using an empirical correlation. There are two that I know of: one is included in ProMash, BeerSmith, the MoreBeer spreadsheet, etc. I recently developed the other. Everyone I know of who's tried both has found the standard correlation to be less accurate.