With Promash you don't need your OG. You only need your FG as read by hydrometer and refractometer. Promash will calculate alcohol by weight and volume, Original Gravity in SG and Plato, resudual extract in SG and Plato, and attenuation as a percentage-both real and apparent attenuation.
Check out what the Promash help file has to say abut the accuracy of this method:
After fermentation, a brewer can calculate the amount of Alcohol present in the beer, the residual extract and also backtrack to the Original Gravity. These calculations can be more accurate than simple determination based on Original and Final gravities. However, the brewer will need both a refractometer reading and an hydrometer reading of the sample beer for this calculation. Note that this calculator can also be useful in backtracking your Original Gravity if for some reason it is unknown to you.
If you have Promash, go to Calculators, select % Alcohol, then click on the picture of the refractometer at the bottom. Input your hydrometer and refractometer readings you have taken
post fermentation and Promash will give you all the results I mentioned above.
A note on taking measurements with refractometers: Even when using a refractometer with automatic temperature control, you should still take your measurement as close to 80F as possible (or 60F if that is what your refractometer is calibrated to). There are two reasons for this. One, the closer your sample is to the calibrated temperature, the more accurate the results. Two, if your sample is very hot, evaporation can skew your results significantly since you are only using a drop of beer. Be absolutely sure your refractometer and sampling tool (eyedropper, pipette, spoon, whatever) are completely dry before pulling your sample and dropping it on the refractometer plate. Again, you are using a single drop and the tiniest amount of dilution will skew your results. Also, I always degas my sample before measuring although thinking about it now, I'm not certain this is really needed. I will look into that. De Clerck addresses use of refractometers in A Textbook of Brewing, Vol 2.
