Problems populating text field from calculated value

Hi,

I am trying to populate a text field based on a calculated value, but I have been unsuccessful.  The logic I'm using is:

• Calculated field equals:  UpdateTextBox(Risk Ranking HSE Impact*Risk Ranking HSE Likelihood)

• Form custom javascript equals:

              function UpdateTextBox(inpval){
                 NWF$('RiskRankingHSEScore'+TextBox).val(inpval);
                 return inpval;
              }

I have used this logic based on advice in a similar forum question, but for some reason it just isn't working for me so suspect there is more to it and I'm missing something.

Thank you!