This usually applies to top ups on your existing automatic transmission fluid. First up there is a need to get an accurate reading from your reservoir and dipstick. Make sure that you park your car on level ground and leave it until the engine is cold to the touch (might take a few hours).
This is to ensure that you get a correct and accurate reading as to the fluid level in the compartment. This is by far the most accurate way of knowing how much fluid to add in an ATF top-up. This is also a good time to inspect the color and smell of your transmission fluid. If the ATF color is reddish or pinkish it’s still good, but if the color is dark and smells burnt, maybe a change is in order instead of just adding fluid.