It's too late now, but I would have boiled up some DME in that 1 gallon you topped up with. That would have keep you from diluting the wort. So, keep that in mind for next time- I always have some DME on hand for starters and just in case I miss my OG.
In this case, you could boil up some DME in some water, cool it and add. If you have brewing software, it could help you figure the exact amount, but 2 pounds DME in a little water should add about 16 points to your OG.