if Max profit = A(x1) + B(x2) + C(x3) + D(x4), where A,B,C is the price of a subscription and x1,x2,x3 is the amount of users, how can I calculate the optimal price while accounting for an inverse rate of change of 1.666% in user base when the price changes by 1%? (1% increase in price = 1.666% decrease in users, 1% decrease = 1.666% increase in users)