Which mathematical constant is commonly used by high-end mathematics textbook and software publishers to determine their 'golden' markup percentage above production costs?

In the world of mathematics education products, pricing strategies often incorporate mathematical concepts to create perceived value. One famous mathematical constant has been particularly influential in premium educational product pricing, creating what marketers call the 'aesthetic price point.' Test your knowledge of how mathematical principles influence product pricing strategies in the educational market.

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Phi (φ ≈ 1.618), the Golden Ratio - often used as a 61.8% markup
Pi (π ≈ 3.14159) - often used as a 314% markup
Euler's Number (e ≈ 2.718) - often used as a 271.8% markup
Feigenbaum constant (δ ≈ 4.669) - often used as a 466.9% markup