Brooder plates often say to use in temperatures above 50 degrees.
Chicks do fine with access to temperatures at and below freezing, as long as they also have a spot to get warm.
So 70 degrees is probably warmer than you need in much of the brooder area.
I assume the chicks are being shipped to you?
I would probably use a heat lamp (yes, the big red kind) for the first few days. Put it somewhat near the brooder plate. Make sure the feed and water are in that warm area for at least the first day.
When shipped chicks first arrive, they are cold and hungry and thirsty all at once, they do not know what things are good to eat and drink, and they do not know that the brooder plate is warm. But they are attracted to light, and when they go to the lit area they find warmth and food and water all in one place.
I think using a heat lamp for at least the first day makes a real difference in survival of shipped chicks. (Of course the brooder still needs a cool area so they can get away from the heat if they want to, even that first day.)
I'm not sure how soon to remove the heat lamp-- maybe on day two or three, maybe at the end of the first week. It would partly depend on how the chicks are doing, and partly on the exact weather conditions that week (remove it when the temperature is stable or rising, not when the temperature is dropping sharply.)