Marking the faucet wouldn't work for me because my hot water tank is a 30g and as I change several fish tanks the hot water in the tank is used quickly and new water is being heated. That means the "hot" water will be hotter. Also, as the hot water leaves the tank, the new cold water coming in is lowering the temp of the water in the tank, so as it comes out it is cooling. I live alone so no need for a larger tank, and for normal activities like a shower this is not even noticeable. But if I turn on the hot water tap after the laundry, or the fish water changes, the hot water considerably hotter than if I run it before. And even during filling half the 90g, the water temp entering the tank will noticeably cool; if i am working in the tank during the filling I can easily detect the water stream cooling down.
To the question of the temp for a water change, for most fish it does not need to be exact. The water temperature in the tropics varies a great deal each day/night, and during rain, etc. A water change is to the fish a rainstorm, and they "expect" cooler water. This is what induces them to spawn following a water change. Interestingly, I just did the weekly change on half my tanks, and within 15 minutes of filling the 90g my beckfordi pencils were scattering eggs by the dozens. We also happen to have quite a low pressure system overhead today, and steady rain, so that affects them too, but it is the water temp drop that triggers spawning.
Anyway, the temp in the tank can fluctuate by a few degrees due to the water change with no harmful effect for most fish. Cooler rather than warmer is better.