The first time I had my name used in any sort of official capacity was the day before I came out to everyone, and the date on my deed poll. The admin at work knew as we'd been coordinating how all this was going to happen, and they'd booked me a train ticket to get back from a routine medical. As I wasn't yet out, on the envelope, and in the enclosed letter to me and the other person going explaining where and when we had to be places, was the name I was born with, however on the ticket was "Mr A C----". The women who printed off the ticket happened to be looking out the window when I opened it and saw my reaction to seeing it. It was amazing in that they didn't have to do that for me, yet they still did.
The first time I wrote it was the day after, the first day after I'd come out, and I was doing some technical drawing. I asked if I could finish it off after we had our morning break as I was unsure whether I was supposed to be putting my name to things, or whether it would cause issues with certification. He just looked confused and said, "As far as I'm concerned your name is Alex," and that was that.