There is a formula thAt allows you to measure fermenting wort. I however don't know it since I use 'beer smith' which has a function for that. I have heard that it isn't very accurate, though. I always use a hydro once yeast has been pitched.
Just google refractometer calculator. I've used my refractometer for a year now. It is very accurate and allows for constant testing of the beer with minimal risk. I pulled hydro samples the first 6-7 batches to verify its accuracy. Make sure you account for temperature on both OG and FG, it makes a difference.
I ahve used a couple of calculators that I found by googling. They gave ma an idea of what the gravity was post fermentation but I didn't find them to be terribly accurate.
I've found the calculators to be fairly accurate, usually within a couple of points. I just use mine to give me an idea of where I'm at. I take the "official" FG reading with a hydrometer.
The app "Brew Pal" has the alcohol correction for refractometer's and hydrometer corrections, amongst other goodies! I use an iPhone, don't know about Android or Blackberry.
I use this spreadsheet from Morebeer http://morebeer.com/learn_vids/vids_refract
The link to Download in under video. I know alot of people say it's not accurate with alcohol present, but when I double check it against hydrometer its allways correct.