Trans fluid question

How exactly do I know if/when I should add transmission fluid? The dipstick just has two lines, so I assume as long as the red falls somewhere between
the lines it's alright. Am I right on this? I don't want to overfill it.
Second question: What kind of trans fluid should I use? I noticed some of them have different substances, but which ones are okay to use in a 300CE? I don't have a service manual to ask. Thanks!
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