Northwestern RD is really tough and hard to predict. NU admits >50% of its class in ED. Moreover, unlike a school like Penn where legacy advantage applies only in ED, it does favor legacy in RD. Thus, many of the RD slots go to legacy, minority, Pell grant qualifiers (whose percentage NU is trying to increase), and so on.
You have a good record and you should be competitive for RD. If you are really interested in NU, consider applying ED.