It is good to start at the Home Depot only about 3 miles from Whites Ferry but make sure you leave at 10:30. That will give you time to get to the ferry, cross over, meet up with the Marylanders, and drive the direct route or twisty, hilly, country roads of western MoCo to the park... only ~3 miles longer than the direct route.
I'm still working out the bugs with Michael but here is a PROPOSED slightly longer but more fun route from the ferry to the park: (23.0 miles from the ferry to the park.) Click on the "Include Large Map" box to see the map.
http://maps.google.com/maps?f=d&source=s_d&saddr=White's+Ferry+Inc,+3,+Maryland&daddr=39.15416,-77.41613+to:39.1775039,-77.3855037+to:39.165584,-77.338739+to:39.1538329,-77.3409547+to:39.16335,-77.28481+to:11950+Clopper+Road,+Gaithersburg,+MD+20878&geocode=FYd0VQIdcSph-yGut0k_O9oWHCmPOf_T2SK2iTGut0k_O9oWHA%3BFfBxVQIdPrli-yndouGDGCW2iTEF_QHds_HX5Q%3BFR_NVQId4TBj-ykVzwluhCW2iTEmiyeoS8a6IA%3BFZCeVQIdjedj-ymfV4A6Gy-2iTGQj-EQarWzHQ%3BFahwVQId5t5j-ymxBgAEry-2iTEnQ2HbCR5XYA%3BFdaVVQIdNrpk-ylnD_d6Kiy2iTHXsZAFmKlnOA%3BFVFkVQIdKkRl-ynRcuLVrS22iTGHut9dK6zhLQ&aq=0&oq=whites+ferry,+maryland+&sll=39.156687,-77.307186&sspn=0.040199,0.078363&vpsrc=6&gl=us&hl=en&mra=dpe&mrsp=5&sz=14&via=1,2,3,4,5&ie=UTF8&ll=39.165472,-77.333965&spn=0.040194,0.078363&t=m&z=14&layer=c&ei=TdRTUJycIejTxQHsjIGoBA&pw=2