How we calculate Defect Leakage in Software Testing?

Assume,

CD = No. of Defects raise by customer after release
DR = No. of Defects raised by Tester before release
ID =  No. of invalid Defects from Defects raised by tester before release(Eg. Duplicate, CannotFix, error in test environment etc.)

Formula to calculate Defect Leakage:

Defect Leakage = [CD / (DR - ID)] * 100

For Example:

If after release client found 9 defects, during the test, tester reported 201 defects and from these 20 defects were invalid, Then,

Defect Leakage = [ 9 / 201 - 20] * 100 = 4.9%