Take the BB out. You're going to need to do it to put the new one in anyway, and it's not like you can ride the bike now. Then you can measure it with any old ruler. Also see if it's symmetrical.

assuming both old and new BB are symmetrical, your new one would need to be 20mm less, because they are sized by overall width, so you have to double the change to figure the movement of one side. However, you need to torque it down to accurately figure out the current chainline, though you could just knock off a mm or 2