I guess I make mine as Very Light, maybe even a little less than 0.5 oz of
vermouth (Very, Very Light?). There is probably a point where the vermouth
decreases to such a point that you're left with bitters and whiskey and it's
not a Manhattan anymore. I don't know what that point it is but if you're
applying the vermouth with a mister or eyedropper you're probably there.
I also like mine with extra bitters and no garnish of any kind.
I think that as long as there's whiskey, sweet vermouth and bitters in some
reasonable combination it's a Manhattan.
And in the interest of full disclosure I am a wannabe bartender, an urban
planner by profession, landscape architect by education and wise-ass through
years of practice.