For me, I would definitely consider 50-60 RPM on the hills too low. What cadence do you usually run on the flats? I'd suggest trying to get within 5 or 10 of that number while doing climbs. Just be sure to keep the cadence up when you switch to a lower gear and don't ease up unless the pace is too high (i.e. you're going 15 mph at 60 cadence, switch to a lower gear, cadence shoots up to 70 but then you ease up because it just got easier so your rpm goes back down to 60 and now you're at 13 mph).