Fixed pelmet - how big should it be ?

Morning gransnetters - does anyone have technical knowledge about pelmets and curtains please?

The ceiling in my living room is being replaced as part of a renovation project. This gives me the opportunity to get a fixed pelmet installed along the length of one of the walls which is three patio doors. A simple pelmet like this is my preferred way to finish off the tops of the curtains. The plasterer will build it, but has asked me how big I want it. I have no idea!
The curtains will be full length. My guess would be that the pelmet should project about 200mm from the wall (to accommodate the folds of the curtains), and the height of the pelmet should be one sixth the length of the curtains. Does anyone know if these dimensions are reasonable?
Many thanks in advance.