Honestly, I don't think the dealership guy knows what he is talking about. The first number is about width and the second about sidewall. You don't usually subtract one and then add to the other.
If you want to fill the wheel wells more, you don't want a bigger sidewall, because that would probably look goofy IMO. The best way is the to get shorter springs, or if not, bigger wheels. However, if you drive something like a SUV, bigger wheels might end up with the "bling" look which would also look goofy
