Because fancy shaped ideal cut diamonds are not symmetrical like round diamond engagement ring, a broader range of parameters are needed in regards to proportion.
Usually the Tiffany Style Diamonds rough will be cut into whichever shape that requires the least amount of sacrifice. Marquise and oval shapes should have a length thats exactly twice the size of the width. If they are too narrow or too wide, the stone will loose light and appear too dark.
When fancy diamonds are cut too shallow, light leaks out of the bottom, brilliance is lost and the diamond engagement ring appears watery, glassy and lifeless.