Read through this thread and didn't really see a specific answer to my question, so wanted to shoot then.
I plan on doing most of the Texas stage races this year which all have TT's that are mainly <7-8 miles. Last year I rode them on road bike with some clip-on aerobars, team skinsuit, and a used aero helmet I was able to get for cheap from an old teammate. I finished top 1/3 in most of them. This year I started training with power. With that being said, it seems that for a typical 40k TT, one would aim for their FTP (in general, probably +/- a few %). Without numerous test rides of actual TT race distances, is there a standard guideline to how to scale that number to shorter TTs? i.e. for just a 10k TT, should I am for 135%? 150%? etc.