Notes by Lex Toumbourou

The 50% rule states that you should plan such that if 50% of your budget were removed, you'd still have a shippable game. Then use the rest of the time to make it great.

[@schellArtGameDesign2015a] (pg. 112)

Later in the book in Chapter 13, he mentions that half the development time should be spent balancing the game. Balance is a clear part of what makes a game great.