Data Controls :: Calculate Textbox Value Based On If Condition Inside Stored Procedure

I have 3 Textboxes in my Web page. When user key in value in these 3 textboxes,their result should show on a "Label"

1) Gross Amount
2) Taxable Amount
3) Tax Amount

I have to write a Stored Procedure for calculating the value of above textboxes.

Condition is:

If Gross Amount is > or < 0.00, then Taxable and Tax amount should calculate based on below formula:
@TaxableAmt = @GrossAmt / @Rate
@TaxAmt = @GrossAmt - @TaxableAmt

but If Taxable Aount is > or < 0.00, then Gross and Tax amount should calculate based on below forumla:

@GrossAmt = @TaxableAmt * @Rate
@TaxAmt = @GrossAmt - @TaxableAmt

and for "Tax Amount" I have 2 different conditions:

If Tax Amount is = 0.00, then Taxable and Gross amount should calculate based on below formula:
@TaxableAmt = @TaxAmt / (@Rate-1)
@GrossAmt = @TaxAmt + @TaxableAmt

but If Tax Amout is <> 0.00, then whatever the value of Tax Amount is, it should key-in/show as it is.

i.e., @TaxAmt = @TaxAmt

For this requirement, I wrote below SP:

ALTER PROCEDURE [Invoice].[usp_tbl_TaxCode_Update]
-- Add the parameters for the stored procedure here
@TaxCodeID nvarchar(50) ,
@GrossAmt decimal(18, 2),
@TaxableAmt decimal(18, 2),
@TaxAmt decimal(18, 2)

[code]...

But its not working as per required.