It is true that a work of writing usually benefits from being trimmed: it strengthens and concentrates the message. However, the ‘cut 10%’ rule of thumb is just plain silly if, like me, you under-write rather than over-write (I generally have to add, add, add before I can even think about cutting). And the idea that once you hit the magic 10% of your word count, you’re suddenly finished trimming is also unrealistic.
This rule is better described as: every word should be doing a job. This is related to the other rules and means that if you’ve used three words to describe one thing, maybe you should be working harder on picking the right verb or noun instead. Maybe you should be avoiding qualifiers like ‘quite’ and ‘kind of’. Maybe you should be avoiding repetition or thinking about whether the reader really does need that detailed description of the door knob.
Every sentence should be doing a job. Does that sentence or paragraph, that flashback or daydream or musing add to the characterisation, the world, the plot, or is it just that you developed this background for the character and you want the readers to know about it, damnit.
And do you need to tell the reader something six times, or will three times do? Will maybe just the once do, and you can trust your reader to know that if you mentioned it, it’s important? Can you perhaps even trust your reader to get the idea if you don’t come right and say it, because of how well you’ve drawn your characters and their emotions and relations?
Sometimes in the effort to get a description across, you might describe the same reaction or feeling two or three times using different imagery. But one well-chosen consistent image will be so much stronger than stringing together a series.
In the trimming process, don’t aim to cut 10% of your word count. Aim to cut every unnecessary word, whether that amounts to 5%, 20%, 50%…