Sensor location and relocation

Well, so far I’m happy with how the installation is going, but I do have a problem. When I installed a contact sensor to one of my doors I placed it with slightly more than a 1/2 inch gap. First, is this a bad thing? Will this cause the sensor to fail? Second, if I have to relocate the sensor, how do I remove the sticky tape without damage to the door? Any help would be greatly appreciated.