Well to start, a "10-20 degree angle" is a really wide range. For reference, here's a 21 degree hill in Seattle: Linky. If you can get up your hill with 48x17, I doubt it's that steep. If it is actually that steep, then you'll have to consider the length of the ride you're doing, and just walk that portion, or find a route around it. If you're able to ride it with what you've got, then just consider whether you're comfortable with however hard to have to peddle (keeping in mind your shape, and that it'll get easier if you keep doing it).

Personally I use 46x17 which I've found just perfect for the fairly hilly (but not super steep) area of north Seattle (and fairly fast on the flats), but I do have to walk if I hit some super steep hill (not very many, most hills are fairly short and I can just power through it).