Jabba,
This is also in that same document, Intensity guidelines for Triathlon. 95% of average 20-minute HR.* So, in your case, you’ll just do a 20-minute TT and take 95% of both average HR and Power.
If you have not yet established your TP, you can find your LTHR independently through a time trial. Begin with a warm-up that consists of 15 minutes of easy jogging with a few 15-second surges at the pace you intend to run for the time trial. Next, increase your effort to the highest level you feel you can sustain for 20 minutes and hit the lap button on your heart rate monitor watch. At the end of the 20-minute time-trial, hit the lap button again. Your LTHR is 95% of your average heart rate in beats per minute (BPM). So, for example, if your average HR in the 20-minute time trial is 170 BPM, your LTHR is (170 x 0.95 =) 162 BPM. Or, enter your 20-minute result into the appropriate field in the 80/20 Zone Calculator.
David
*Matt and I are considering changing this to 100% of 20-minutes. With Masters athletes in particular, we’re finding that they are having trouble getting HR up fast enough.