The best way to estimate miles to go is from the finish for the day to Fort Lee, NJ, where the GW Bridge crossing is. From there to City Hall it's 13 miles. If she runs around in circles in between, that throws off your estimate if you are calculating miles completed by Pete K minus miles ran by Sandy.
So far Sandy has run over 15 miles more than Pete K to get to where she is. For instance, Pete K ran 306 miles in CA. Sandy ran 314.
Google Maps is the easiest tool for estimates. You can also put the route into Map My Run like Coach Dave did to show Sandy's route estimate per state. If you compare the two, it's pretty close plus or minus 1/2 mile.
Side note: New York Marathon is Nov 5th so there will be 50k running fans and media in town to celebrate a record smashing finish.