First of all well done on getting through the Command training. Its up to you what you want to call yourself but it is not a Queens commission in the military or an indication that your skills might need to be called upon in the case of a medical emergency. It is a rank within a flight deck so that the crew knows who the PIC is. Beyond that it has little value. Anyone who is no longer an active pilot but still wants to be referred to as Captain is doing so for purely egotistical purposes IMHO. I have used it in company emails but mainly when communicating with 3rd parties on behalf of the airline to people who might feel that the position of Captain has some importance.