The answer is maximum efficiency. Milady states this directly: maximum efficiency should be the primary concern when constructing the best physical layout for the type of barbershop you envision.
The book gives a concrete example — a low-budget barbershop offering quick service needs several stations arranged to move clients through efficiently. Costs, personnel, and retail sales are all real considerations in opening a shop, but the specific factor the book names as the primary concern for physical layout is efficiency.
Milady Standard Barbering, 6th ed., Ch. 21 (The Business of Barbering), p. 768: "Maximum efficiency should be the primary concern when constructing the best physical layout for the type of barbershop you envision."
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