' Those are my feelings, and I don't see any rule emerging; just a style point that leads me to avoid juxtaposing two 'that's ('that is, that'), which seems rather awkward. If there is no following 'that', then I prefer 'that is', which is more expected, I think. Otherwise, I would think that 'namely' tends (of course) to name, while 'that is' tends to explain.
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