OK...
If we assume... NEVER ASSUME... still if we assume that we have the right city, river and bridge... We travel West / left - from the Old Ford on Anduin All the past Rivendell, Amon Sūl and most of the Shire. Then we stop when we're on a North/South line with
Eryn Vorn "The black Wood". From there we turn towards the Northeast and travel another 150 miles until we get our feet weet in
Lake Evendim 
.... is that it?