I did a flat TT once on my track bike. I just had to make it look like I had a brake. Heres what I did.
Take a look at the course speeds in the past. Using a gear chart figure out what gear you can do that speed in with a cadence you can hold no problem. Factoring in of course the "roll over" effect you get from a fix gear.
For instance, the course I did had a winning rider doing 45kmph avg. It was also short. I picked a 90" gear done with a 50x15 for increased efficency. The only spots I was slower was in the start and the turn around.