This is my understanding:
Week 1: 95
Week 2: 90
Week 3: 85
Week 4: 80
Week 5 (and beyond if fully feathered): 75 or outside
If they're still not even 4 weeks old, they will need more heat than 70 degrees. Of course every brooder is different, and if they're being raised in a house with heating, they might need less. I agree that this is a guideline (above) and you should let your chicks tell you with their behavior, but the above is a good place to start, and they very well may need closer to 85 degrees.
I would keep the light on a night for another week or so - but that's just me. I would also use a red bulb as that will keep the pecking and cannibalism at bay, and will allow them rest at night. By the time mine were 3.5 weeks, I was turning off the (red) light in the evening for 3-4 hours while I was still up, but always put it back on at night because I didn't want to wake up to frozen chicks! They had a big enough brooder they could completely go away from the light if they wanted, and I found that 5-6 would huddle in a different part of the brooder, 3-4 would be right under the lamp, and another 5 or so would be dispersed throughout the brooder (the brooder had separate "rooms"). Goes to show that they all have different preferences!