Help with a script

Hi everyone,

I have a Conversion Numeric  filed with default: 1

I have a  Numeric field named: amount

A DDL named AddTax with 2 items(YES and NO )

On change event I have the script:

switch (xfa.event.newText)

case "YES":

tax.rawValue = 0.13*conversion.rawValue*amount.rawValue;

  break;

case "NO":

tax.rawValue = 0.00;

break;

At last I have another numeric field (Read only) named: tax

Everything works fine but in the case someone enters date to all fields but at the end

if has to change the conversion value from 1 to 2 for example, the value on the tax field remain the same!

I have to select the yes item in the DDL to update the tax.

There is another way so the tax field is always updated in case they change afterword the conversion field?

Thank you