ok, here you go.
the official 'ticketed' area is along the Zig Zag road from the bottom of Box Hill up to the top, from what I can work out from the official guff, it ends around about opposite the viewing point, with the field behind the viewing point being used as an area for food, bogs and a big screen to watch the rest of the race (for ticket holders)
by my guess, either just to the west of the 'smith and western box hill' or certainly to the east of that point should be public viewing. I certainly hope it is anyway as that is where we are intending to walk up with provisions from our house which is about two miles or so south of there.
http://maps.google.co.uk/maps?q=box hill&hl=en&ll=51.250151,-0.293584&spn=0.014398,0.042272&oe=utf-8&client=firefox-a&hnear=Box Hill&t=m&z=15
eta - if you look at the link here, the yellow pointer on the map on this site points to roughly where the viewpoint is and the field behind it is where the ticketed area i mentioned earlier is (and in theory, ends)
http://www.nationaltrust.org.uk/box-hill/
just in case you don't know, Box Hill Road is long and it continues down to Headley Common Road, where the riders turn left and head over Headley Heath
http://maps.google.co.uk/maps?q=National Trust Box Hill&hl=en&ll=51.261029,-0.26144&spn=0.014394,0.042272&sll=51.286202,-0.294392&sspn=0.460372,1.352692&t=h&hq=National Trust&hnear=Box Hill&fll=51.255765,-0.273156&fspn=0.014396,0.042272&z=15
along the Headley Heath Road would also be a good viewing area, but the way the car parking and/or train access is being set up, its mostly from the South (Dorking) so would be a big old walk if you were coming from that side.
the route turns LEFT at Church Lane on this next link, from their down to where it begins the loop again is pretty narrow, banked lanes and unless farmers are opening up fields, i can't see it would be the best to view from
http://maps.google.co.uk/maps?q=National Trust Box Hill&hl=en&ll=51.270642,-0.270882&spn=0.014391,0.042272&sll=51.286202,-0.294392&sspn=0.460372,1.352692&t=h&hq=National Trust&hnear=Box Hill&fll=51.268145,-0.270452&fspn=0.014392,0.042272&z=15
hope this is helpful, apols if you know all this already. if you want to know owt else, i will try and answer, am very local