I would have added it before the boil just because I hate putting the hydrometer in boiling wort, probably wouldn't crack if you warm it up first but I don't want to give advice on doing that. What the hell, throw it in if you want a higher SG or just go with what you got. You can check it later to see what you got. My thought was add until you got what you wanted rather than trying to calculate what you need. You have a decision to make but do it soon.