Nickname based on my first name. My legal first name is only used when I sign things. Otherwise, it’s my nickname–my nickname is even listed on my business cards.
I selected first and middle … Through most of my life, I’ve used my first name; but I always have used my middle name online, and it’s started to phase in as more of my IRL interactions are with people I first met online. Easier to keep with the name people are familiar with, and I’ve always liked my middle name better anyway–it’s just hard to make a break with the past practice. So if I ever turn up at a Dopefest, I’ll introduce myself as ‘Christian’.