The point about checking the topic page to see how many people follow it is a good one. I discovered this a while back, but tend to forget.

I do suspect, though, that going for the broadest tags to increase distribution chances can in turn bring down your read ratio, as your story may be shown to more people who find it less relevant to them.

For example, 'psychology' is a popular topic - 4.3 million followers - but if I use it for a piece which addresses psychological matters only incidentally, I might find that lots of people looking for a more specific and academic approach to the subject give up.

I've seen specifically that this topic tends to crop up in the analysis of readers of a quite a few of my pieces, but I never actually write anything focused on 'psychology' per se, just indirectly on situations and our reactions to them.

Without knowing more about how the algo works, it's very hard to second-guess which tags will do best for each piece, I fear.