I needed fewer than 13 bits of evidence: http://lesswrong.com/lw/fp5/2012_survey_results/

I likely committed some level of base-rate fallacy though (regardless of what the truth turns out to be). Trans* is more available to me because I hang out in queer communities, and know multiple transgender people.

The username contains more than 13 bits of information (being 14 characters long) so this might not be too unreasonable.

Transsexualism seems way overrepresented in geeky circles: off the top of my head, I could think of seven MtF and two FtM transsexuals within my circle of acquaintances, and there might be a few that I'm forgetting. LW definitely matches the definition of a "geeky community", so assuming a relatively high base rate would have been reasonable to me, based on my experience.