Choose something less than 2 times the capacity. In your case two times 600 equals 1200 mA or 1.2 Amps. Choose a setting close to 1.2 Amps and a totally discharged pack should take about 30 or 40 minutes to charge. If you choose a higher setting, it will charge faster but is harder on the pack. If for some reason you are in a big hurry, you can choose 3 times (e.g. 1.8 amps) and it should charge in only about 20 minutes. But I generally don't go more than 2 times for NiMh.