Well as you said you have not been precise. There is a lot to be said about the method of not being percise. It's much more relaxing and for many, SG is on overkill placed real high on the importance list. You added some more malt to it so it will certainly replace what was lost. In the "Joy of Hombrew" towards the back (don't have the book with me) there is a chart that tells you how much of ingrediants to add to change the SG (here I mean Specific Gravity) so many points. Eh.... it's going to be great
The OG, or Original Gravity, is taken when you put wort in it's primary. This can be called SG, or Starting Gravity. Many use SG to mean Specific Gravity, though using this does not say much as to when it was taken. Then before you bottle you take the FG or Final Gravity. Your alcohol percent will be OG - FG *100. Many take a gravity reading when the fermentation activity stops.... then take it again in a few days to make sure the fermentation has indeed stoped before bottling (thus preventing HB granades).