The market for low-quality cellphones has a demand curve given by P = 108 - 0.01Q, where P is the price per phone in dollars (and cents), and Q is the total number of cellphones demanded. The market supply curve is given by: P = 8 + 0.04Q.

Suppose the government decides to introduce an excise tax applied to low-quality cellphones (previously they were exempted from taxation). If the goal of this tax is to maximize the resulting revenue from the tax, then the "per unit tax should be set to [TaxRate]".