Hi NFamato,
It sounds like you have only taken the 1 gravity reading 12 days into fermentation - is that correct?
In order to calculate an estimated alcohol content, you need to take a gravity reading before you pitch your yeast and again once when fermentation is done. The difference between the first gravity reading (the original gravity, OG) and the gravity reading taken after fermentation is complete (final gravity, FG) can be used to estimate alcohol content. You can search the web for a calculator, but an estimation you can use is ABV = (OG - FG)*131.25.
I think the alcohol percentage of 2.4% you are getting is potential alcohol based upon the sugar currently in solution in you beer. This scale on the hydrometer can give you an idea of your alcohol percentage if you take an OG and FG reading. When using this scale make note of the potential alcohol before you pitch your yeast, and again when fermentation is done. (for example, if you get a potential alcohol reading of 6% before you pitch and 2.4% after fermentation is done your estimated alcohol percentage is 6% - 2.4% = 3.6%)
So, if this is the only reading you have taken you will not be able to determine the alcohol percentage of this brew. However, as Pezman1 mentioned, you will be able to tell if fermentation is done... if you hydrometer readings do not change over a few (3-4) days, fermentation is done.
Hope this helps,
-Schmutzie