Have you read this article?
I would just target 140 HR at the beginning of the test.
After 10-15 minutes of warm-up, if your HR is 140, you can start the test. If in the next one hour your heart rate drift is less than 5%, then your AeT is 140.
If the heart rate drift is more than 5%, then redo the test with a lower target HR of say 130.
After a couple of tests, you should have an estimate of your AeT.
I am assuming you are using a chest strap to track HR.
I hope this is helpful.