Taking a stab at this...

I assume that %FORM is key/value pairs of the returned values from the form. Unfortunately, if a value is not set, it is not passed, and very likely will not have a key/value pair.

Your best avenue is to create your own @param_list of 'must have keys' and determine if they are not present. If they are not, then you need to have code determine the best default value for those unpassed parameters.