smart vent in 1890's Victorian, outside NYC
An alternative is to install a "ventilated nail base"http://www.atlasroofing.com/tabbed.php?section_url=51
It is basically rigid foam, an airspace, and a new deck which is attached over your existing deck. More $$, but helps you insulate as well.
In the instructions for the SV it says to stop it 12" away from dormers, valleys, rakes, etc... which makes sense as those spots tend to gather quite a bit of water.
The ventilation split ideally would be 50/50, but it depends on your local code. Mine (Canada) calls for 1:300 vent to ceiling area, with a 25/75 split top/bottom and vice versa