For 99% of folks, doing this test above AeT (top of Z2) will result in a HR drift of more than 5%. That’s how you know. I would also recommend doing the AnT test. It may well be that your Z3 is getting smaller (a good thing) as your AeT moves up. In that case Z3 may only be 12-15 beats wide. So the effort of sustaining AeT is not much different than middle Z3. But if Z3 is small then you need to drop Z2 and do all base training in Z1 and add some Z3-4 into your plan. This is all described in the new book.