Honestly, I'd say your best bet is to watch it from Main Street, catching the end of the parade from directly left or right of the train station (be on the left facing Cinderella Castle if you want the closest exit to the monorails). FoF moves fairly quickly (for a big parade), and there really isn't a way out that doesn't put you moving behind the parade that also doesn't throw you into the pre-parade waiting crowd if you're watching from Frontierland. While there are crosswalks, they often get fairly congested, and back-ups happen frequently, especially as the parade is passing certain points of visibility along the crosswalks. Plus, some of the Main Street crosswalks have to be closed at a certain point to maintain safety throughout the parade, leaving you even fewer exit options. Instead of trying to follow a very particular, very small path to get out maybe ahead but probably at the same time as the Main Street crowds or behind them, I'd save myself the headache and just watch from Main Street.