50% Rule for Game Development
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.
Schell (2015) (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.
References
Jesse Schell. The Art of Game Design: A Book of Lenses. CRC Press, Boca Raton, second edition edition, 2015. ISBN 978-1-4665-9864-5. ↩