Hi,
As per my understanding, the function set_priority( matrix, impact, urgency) in apc.js should be the function to set the value of priority.
Note the following lines:
new_pri = matrix[idx];
var pri_element = document.main_form.elements["SET.priority"];
if( new_pri != "" && typeof pri_element != "undefined" )
{
pri_element.value = new_pri;
There is other function in apc.js to generate the matrix, but including the severity in matrix may be too complex.
It could be better to add some logic after the above lines, set the pri_element.value again if certain condition is met.
NOTE1: the pri_element.value is the enum of pri, not the sym of pri
NOTE2: the above is just my understanding, and for your consideration only. I never test it.
Regards,
Mark