First week with the babies, put the pad on the highest setting. The second week, put it on the medium setting, and watch them. If they huddle inside in a tight clump, turn it back to high.
The frame should be adjusted so the pad touches their backs. They double in size each week, so you will need to adjust the frame so it's as tall as the chicks' backs. Do it again the third week, and lower the setting to medium or low.
If the room where you have the chicks is warmer than 70F, the pad doesn't need to be as warm as it would if the room is kept at 60F. In fact, the cooler the room, the better off the chicks will be because they will adjust to cool temps very early, and you'll be able to move them into the coop with no heat by five or six weeks.