From what I've seen, in context, a whataboutism is concerned about the argument of moral authority and precedent from the person accusing a AES or progressive of something bad...
Here's some rules:
Apply it when it relates to the precedent of incidents that seem exceptionally bad, when committed by Actually-Existing-Socialist countries, but are ignored, if not justified in Capitalist ones... they could be explained as historical thinking of the time or justified under circumstances
For example: https://hexbear.net/post/2329752 (Homosexuality laws compared between historical Soviet Union and the West, lemmygrad meme)
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However, don't use irrelevant ones to detract from past mistakes, they make you look daft and aloof
I'll explain more later on...
Yea don't worry about overusing it it because no matter how perfectly you use it they'll just dismiss it.
You can point out the fact they're being shameless uninformed hypocrisy, literally demonizing somebody for something while they personally help enact a more egregious version of that thing, and they'll stay say your criticisms are invalid.
They'll knock you over the head, rod you blind and take all the clothes off of your back then turn around and condemn you for stealing from their garden. If you point out you had to because they robbed you blind and you'd starve otherwise they'll call it whatabaoutism.