Forcing recalculation of field after check box selection changed?

There come a time when one must admit that help is needed.... I'm there.
 
I have a form I am developing which will calculate the cost of an event based on various factors - some directly entered as numbers, other choices being made through check boxes. The check box choices will determine which calculation fields are used (the check boxes determine which types of spaces are to be used for the event, thereby setting which costs should be calculated.)
 
I have gotten this far:
 
(getField("cole").value == "Off")
? (event.value = null)
: ((+getField("evnthrs").value < 5)
? event.value = 5
: event.value = +getField("evnthrs").value);
 
This script allows the Hours of the event ("evnthrs") - this space "cole" has a 5 hour minimum charge, hence the comparision - to be placed in the field to included in calculating the total, and it works in that it calculates and assigns the value once the "cole" box is checked.
 
My problem is this - it doesn't update and remove the value if the box is later unchecked.
 
I'm fairly certain that I am just missing something in my efforts to find something in the documentation about how to force the fields to recalculate... Can someone please tell me where it might be?
 
Thanks!