In principle, you should use:

(1+Return%)/(1+Tax%) - 1 = AfterTaxReturn%

For small numbers, your approximation is close enough. E.g., 1.04/1.003 - 1 = 3.69% instead of 3.7%.

For larger numbers, the difference is larger:

1.04/1.00505 - 1 = 3.48% rather than 3.495%