It's going to depend a lot on length of the TT. Your FTP is probably different on your TT bike than it is on your road bike, too, so I'd check into that, maybe do some field tests or something.
From experience, I know about what I can do on the TT bike for 10 min, 20 min, 40 min, and an hour, which seems to cover most TT's. When I start, I go a bit under that under (depends on the length again) for about the first half. The next 1/4, I'll usually go just a bit above the number (depending on how I'm feeling), and for the last 1/4, it's pretty much just all out.
Again, you kind of need to experiment and find out what works for you, though. Local practice TT's are muy bueno!