Hi Robert,
you can get a good approximation from doing rides like this time trial or other effort ( I like to go to the Crit Course in Plano, circular mostly flat) and then using a site like
http://www.kreuzotter.de/english/espeed.htm (my favorite)
http://www.analyticcycling.com/ http://www.mne.psu.edu/lamancusa/ProdDiss/Bicycle/bikecalc1.htm http://swiss2.whosting.ch/mdetting/sports/cycling.html The chart above makes use of 4 power zones to build up a profile. The zones Usually refered too as CP0.05, CP1, CP5, and CP30 or something like that. A TT specialist is good in the CP30 zone but probably not CP.0.05. My personal profile has always been highest by far on the CP1, dropping down quite a bit on the others. The only way I ever won a race was to use that to my advantage, go too early for the sprinters at the end (high cp0.05s), and do my best not to get dropped by the climbers on the hills (high cp30s).
The problem with this weekends TT was that it is relatively hilly, and we had a good bit of wind. I think it was easily 3.5 minutes harder than "Normal".
The good thing is that any course, under similar conditions, similar bike etc gives you relative comparison based upon your time. Not for nothing they call the TT "The Race of Truth."
p.s.
A single loop of the Tejas TTT course is a good comparison as well.