HTML & javascript form field value calculation

See the webform located at http://jasonmod.bondwaresite.com/webform.php?ViewForm=5 which I created only to use the generated HTML from the browser source.

Can you view the source, look at the HTML and help me to craft the following:
Additional button [Calculate] to be located to the right of "Add 3% Electronic Payment Convenience Fee" and when clicked will calculate the product of "Amount" and "Add 3% Electronic Payment Convenience Fee" and place the newly calculated value in "Calculated Total". Database fields exist for all form fields and are named as seen in the source. I am assuming javascript is necessary to accomplish this. My javascript skill is very limited. I am also assuming there is no reason the javascript will not work if placed inline on the same page. "Add 3% Electronic Payment Convenience Fee" field and "Calculated Total" field should not be editable by user.

Notes:
This CMS system allows HTML to be used where needed. I can copy the HTML from this type of form (which is built using a site tool) and place the HTML into a new site page, edit the HTML so as to reformat the layout of the table, customize attributes such as placeholder text and other changes.

I hope this is enough information. If not, feel free to ask.

Thanks,
Jason

 

Hello All...

Another thought I guess is that javascript could be disabled in the user's browser. The more I consider this, the more I believe that I could do without storing the calculated total especially knowing that results could be skewed based on user's browser. Amount, Item Name and multiplier used in Fee are required. Others are not. However, I still wish to display the calculated total if the user wishes to click the [Calculate] button to see it.

Make sense?

Thanks,
Jason