I used this method for setting up my current training plan. It's working well so far. I used the 30 min TT method simply because I was too cheap to pay the $200+ it costs for the VO2 LT testing. However, I'm doing this test though the Multisport world expo and getting it done for $75! I can let you know how close my numbers were.
How will you know which one is right? A test done inside or a test done in real world conditions ;-)
If done correctly the tests should be about 1-3 bpm different for LT. I have always thought that athletes should have outside LT and inside LT. The conditions are different. Let us know how it goes!