Perhaps, using CHEAP refractometers you have issues. I don't. Mine are dead-nuts accurate time and again. You DO need to make sure you have ATC refractometers, and you've calibrated them recently (to the ambient temp). Once you've done that, as long as you're reading it correctly, you'll get accurate readings that can be adjusted for the presence of alcohol.
I reserve 4 Dram size vials of my OG sample and then pull a matching size for the FG sample, of every brew. This allows me to get readings from both solutions at the same time, recording the readings. I've found that if you don't correctly read either measure, your numbers will be off (on the conversion spreadsheet). It's human error, not the tools there. Blame the monkey, not the wrench.
BTW, rhamilton, "only a hydrometer will give you correct FG readings." is 100% BS... A CHEAP refractometer OR hydrometer is suspect by it's very nature. Get a good version of either and you'll do fine. There are PLENTY of threads from people with inaccurate hydrometers.
With all due respect, what you are saying is wrong.
A refractometer does not measure specific gravity, it measures the refractive index of a solution and usually translates this into degrees Brix which is a representation of the amount of sucrose dissolved in that solution. (1 degree Brix is 1 gram of sucrose dissolved in 100 grams of solution). The Brix reading is then converted to an estimate of S.G. by applying the formula "Specific gravity = (Brix/(258.6-(Brix/258.2)*227.1))+1"
But a beer wort contains mostly maltose, and very little sucrose, and this results in an incorrect Brix reading being reported when measuring wort rather than a sucrose solution. So if the refractometer reads 10 Brix when measuring a pure sucrose solution, the S.G. of that solution will be 1.040
If you make a wort that reads 10 Brix, and measure the S.G. of that wort, you will find that it is closer to 1.038, but the actual gravity will depend on the sugars dissolved in that wort.
If your conversions are "dead-nuts accurate time and again" then that is either because you are making worts with identical compositions, or because you are not checking the results with a hydrometer.
As for the MoreBeer spreadsheat, I put the refractometer readings in from a recent brew where I measured the O.G. and F.G. with both the refractometer and hydrometer. Here are the results measurements I took:
Pre-fermentation Brix 14.75, O.G. 1.056
Post-fermentation Brix 6.75 F.G. 1.013
Here's the results I got from the MoreBeer spreadsheet
14.75 pre-fermentation Brix is rounded up to 14.8 and translated to 1.060 (4 points high)
6.75 post-fermentation Brix reports 1.007 F.G. (6 points low)
Using Promash, the pre-fermentation Brix translated to the measured O.G. correctly.
-a.