You're calling the function from within the function. Or at least attempting to.
Use the getElementById to set the value of final, not calcBMI.
If anyone knows of a website that can offer ColdFusion help that isn't controlled by neurotic, pedantic jerks* (stackoverflow.com), please PM me with a link.
* The neurotic, pedantic jerks are not the owners; just the people who are in control of the "popularity contest".