I've never done a Friel 30-minute test but thought it would actually help me determine some baseline metrics. I discovered I had a LTHR of 175.
Strava and Garmin apps don't show you the average HR per lap (at least not in non-premium modes) but Garmin Connect on the desktop does. Click on the Laps tab.
The first 10 minutes show 1.48 miles in 10:03 (6:48 pace), average HR of 141, and max HR of 175. The next 20 minutes show 2.92 miles in 19:58 (6:50 pace), average HR of 175 and max HR of 184. I ran this by effort and neither checked my speed nor how many laps I was going. Didn't go out 100% but tried to pace it evenly but still at a 90% effort.
Z1-Recovery-114-142
Z2-Aerobic-144-154
Z3-Tempo-155-163
Z4-Sub-Thresh-165-175
Z5a-Super-Thresh-177-179
Z5b-Aerobic Capacity-180-184
Z5c-Anaerobic Capacity-186 (low, no high value given)
Using this data generally speaking, it appears anything from 165-175 can be run for my sub-threshold workouts but I may be 100% wrong here, if Friel's zones aren't accurate, etc. I understand that is a possibility.
Also am curious if anyone has done this and also done the Hadd training (5x2000 tests at various HRs).