Reduce Rate

I need the following:

Reduce Rate

Scenario: Taxable Basis = Line Amount x (Rate x Reduced Rate Percentage)

Example:

Line Amount = 10000

Rate = 5%

Reduced Rate Percentage = -50%

Taxable Amount = Line Amount x (Rate - Reduced Rate Percentage)

= 10000 x (5% x -50%)

= 10000 x 2,5%

Tax Calculation = Taxable Amount x Rate%

= 10000 x 2,5%

Tax = 250

Is there a formula in the tax motor to achieve this calculation?