I had always been taught to begin a descent from TPA abeam the touchdown point, even if tower tells me to follow another aircraft. Yesterday I flew with an instructor who did not want me to descend from TPA until abeam the aircraft I was following. I understand that I begin my base turn when the aircraft I am following is abeam my wing. I did not know to also wait until then to descend from TPA. It does make more sense to stay at TPA until this point but I can't find anything saying one way or the other.
Every CFI I have ever flown with has a different procedure for when to descend. Its annoying. Think of it as descending to the runway when you are in a position to land. If you are not following another aircraft, then you can begin your descent abeam the threshold. If you are following another aircraft, then you should wait to descend until you are in a position to land as #2, which could be after you've already turned base or (long) final.