For early entry, plan to be at the main gates. I would give this advice for any attraction, because I wouldn't trust the Grand Cal. entrance to open on time and have enough staff to clear the line quickly.
The last time I was at DCA for early entry, the rope was just before the entrance to Avenger's Campus on the Buena Vista Street side. The early entry rope was on the right side of the walkway and the standard rope was on the left. This was in May, so others may have more up to date info.