As a Donger main, I should point out that there is a common misconception here:
- If there are no inactive turrets, the oldest will be moved.
- If there is 1 inactive turret, the inactive turret will move.
- If there are 2+ inactive turrets, the one inactive the longest moves first.
- In the event of a tie, like when Heimer B's, it refers to the order placed.
I found this out in a game after I ganked a lane with a couple turrets. I came back to my lane with one turret, placed one down, and instead of only having 1 turret I had 2. I kept closer track of my turrets after that, and these are my findings. Note that I found this all through PVP experience, so there might be some subtle nuance that I didn't notice.
I personally don't feel a counter is necessary, as it's part of mastering the champion. Kinda like keeping track of Zed's shadows. If you TRULY need one, then stop smart casting the ability, and it will count it for you. That said, I smart cast my turrets as I don't have a problem keeping track of them, and recommend you don't rely on it. A counter would be a crutch, and would only hinder play.