I wouldn't worry too much about the classical proportions. Their importance has been exaggerated over the centuries to the point of mythology. Beauty derives from structural clarity, workmanship, and quality materials. This is why so many simple vernacular structures are still profoundly beautiful. Follow the existing proportions of the house. It's why you bought it in the first place. That house was built in the Arts and Crafts era, so Craftsman styling would best suit it.
I just realized this is the Marianne Cusato who wrote "Get Your House Right." Great book!
Crown modeling is the upper portion of the entablature in the Greek and Roman orders of architecture. It's based on the exterior of buildings like the Parthenon or Pantheon. The ogee is known as a terminating molding, and was only ever used to end the order into the ceiling. Yes, you can do whatever you want, but if you want to do it historically correct base your designs on the construction and proportions of one of the orders (i.e. Doric, Ionic, Corinthian, Tuscan, or Composite). There are many subtle and elegant rules the ancients followed that are essentially lost today, such as the use of inflection, hierarchy, and rhythm. These are obtained by intelligently varying the moldings in size, height, or profile in relation to other parts of the room. There is room for variation and experimentation, but it will almost always look better if you stick fairly closely to the one of the orders, where these subtle artistic ideals were worked out a long, long time ago. Ultimately with crown molding we're just creating decoration with shadows, and gradually terminating into the ceiling (as opposed to the abrupt and harsh 90 degree angle).
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