I'm going to attempt a clone of a commercial beer. I already know the hops used and base malt. Obviously I know the ABV too. My question is if I take a gravity reading of the beer from the bottle would that be the FG of the beer as it was produced or would there be a difference eg if there was any bottle conditioning involved. If I have the FG then I could extrapolate the OG and from there work out the grain bill to a fairly large extent given I know the base malt and I can take a fair guess at the colour if any specialty malts were used.
Be sure to measure at 20°C, or whatever temperature your device is designed for. If you are using refractometer, you are going to need some fancy calculations I can't help you with. If you're using hydrometer, make sure it shows 0 in pure water, and shake your sample before measuring,to get rid of CO2 .
Bottle conditioning may change a bit, but if you are willing to use %ABV from the bottle it's no issue. In many jurisdictions it's ±0.5% points anyway. This translates to ±10% of grain in typical 5% beer. With such a wide margin you have to accept, processes like bottle conditioning have little meaning.
Specialty malts are much more than color. To figure them out use taste and, if avaliable, style description.
Obviously one place to look for any hints and tips are the various brew fora and recipe lists. Most commercial beers have a "guessed" list going on somewhere.
The SG of the bottled beer (in most large scale commercial brewery examples) is practically the same as the FG of the brew. Filtering usually removes the yeast and effectively stops further fermentation so little changes as regards the SG of the bottled beer vs the FG of the brew. For "craft ales" containing live yeast and/ or other biological agents the beer can continue conditioning for a while which may result in altered (read lower) SG compared to FG in the brewery.
With that information one could indeed "do the math" and take an educated guess at the malt bill. It would be remarkably fortunate to brew an exact copy on the first attempt but with a few adjustments one can usually get very close. Good luck!
In short yes, it helps
With a SG reading from a hydrometer and a refractometer you can get the OG within 0.001 or so and obviously the FG, both are very useful in replicating an unknown recipe.
Knowing the OG is very important and makes guessing the grain bill that much easier.
Also having an accurate SRM helps a lot with calculating the specialty malts.
Beersmith and Brewzor have the calculators to get the OG from post fermentation gravities.