The "perfect" designation in cocktail parlance means equal parts sweet and dry vermouth, and in the Manhattan this split produces a drier, more nuanced result than the original — the dry vermouth's tartness cuts through some of the sweetness for a leaner, more aromatic stirred whiskey drink.
stirred · cocktail glass · none
add
Combine rye, both vermouths, and bitters in a mixing glass with ice.
stir
Stir for 25–30 seconds until well chilled.
strain
Strain into a chilled coupe.