You can do the 30 min TT test w/o a powertap.
Find a course which is relatively flat. A track is perfect, if you have one available. Warm-up as you would before a short race and then begin the time trial. Start your heart rate monitor immediately. This should be one with an average heart rate mode. The effort of this time trial should be racelike--give it all you have. Ten minutes into the time trial (20 minutes to go) hit the 'lap' button on your heart rate monitor so that when you finish you have your average heart rate for the last 20 minutes. This number is an approximation of your LTHR. The more times you complete this test and observe your heart rate relative to breathing in workouts, the more refined your LTHR will become.
I do this test at Sandy Hook which eliminates having to stop, especially this time of year. Just do it when the winds are light and/or from the West so it is a cross wind. I found this test to be very accurate for me once I got used to it. I have a Power Tap but can't use it now I've gone to disc brakes.
Once you have LTHR use one of the calculators available on-line to calculate rates. I like Joe Friel's:
http://www.performancetrainingsystems.com/newstips/tips_zone_finder.html