Late hydrometer readings?

Does anyone know if it is possible to get an accurate hydrometer reading without the initial wort reading?  I broke my hydrometer while cooking but bought a new one today.  My brew is 5 days into fermentation and looking great but I would love to know what % it really is in the end.  Any thoughts?

-BM5150

When it's done, you could take a reading, and say you got 1.012.  You could use the typical attenuation range of the yeast, say 73%, and back into the expected OG.  If that was in sync with your recipe forecast, and your typical success rate of hitting your recipe forecast, it would be close. 

Thanks for the response!  I'll look in to that.  BeerSmith calculated it at 4.9% so I'm going to assume that's close enough.  This is a Real Root Beer recipe so it's not supposed to be super potent.  How has BeerSmith been on accuracy for you guys?