The practical method is to value the business when each unit is purchased. If business value rises, the unit price rises; if it falls, the price falls. The parties may agree in advance on recognized valuation principles and on the experts who will carry out the valuation.
The essential idea is gradual transfer of real ownership. Every payment should therefore be understood according to its true function—rent, profit share or purchase price—rather than treating all payments as one undifferentiated installment.