right the problem is that you are not evaluating that variable to it's value. Basically the way I have fixed it is to build up a string of the command (as that is what -c is expecting)
so you have
string $com = "resetCamera("+$cameraStartOverscan+")";
then in the button put -c $com and that should work fine.
Bloody mel it's so stupid sometimes!!
Technical Director - Framestore
Currently working on: Your Highness