I think this test looks valid. It would be better if it were truly flat but I think you’d be safe using 135 derived from the Pa:Hr ratio in TP.
If you run this and other courses regularly then you can monitor your times on them running while trying to maintain a 135 upper limit on HR. You should see weekly improvements in speed. After a few weeks redo this same test using 135 and you should see a lower Pa:Hr. If you do then redo the test at 140.
As you point out this will vary from day to day depending on recovery state. If you have ADS then there is no danger in running right at AeT virtually every day. You should recover in 24 from these aerobic efforts. As your AeT Hr goes up and you become less ADS then you will need to do some easy recovery runs and not all runs right at AeT. Once AeT and AnT are within 10% of each other then you can almost completely drop these AeT runs and run either in Z1 (80-90%) or Z3 and Z4 (10-20%).