This is something I do based on past experience because I calculate the outcome from other simliar situations is what is meant by 'as a rule of thumb'. You can think of your thumb as a measuring tool.
I like capturing complex ideas and exhibiting them in a simple way. Languages can be mathematical, meaning that once you have mastered that grammar point, you can pretty much just figure out the right form for a similar given context. Then there are idioms, like the one above, which, by their own definition, don't play by the rules (another idomatic expression).
Is there something that you would like to express in an English idiom?
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