Plain English is conversational English. If you would not use a word or phrase when speaking with a colleague, don’t use it in your writing. Imagine you are writing for your Aunt Agatha, your neighbor or your friends on the train. These people will keep you real because they are very very smart and they will not tolerate fussy, impenetrable sentences. (And if you use fussy, impenetrable language in conversation, you problems are far too deep to be resolved in a writing program!)
Simply say each sentence aloud to edit for plain English and to cure clutter and grammatical errors. If you are smart enough to have made it through law school, the grammatical rules of modern English are embedded in your ear. Saying your sentences aloud is the only tool an educated writer needs for effective sentence-level editing.
And plain English does not mean simple English. You are entitled to use your massive vocabulary. But use it to achieve precision and nuance, not to show off your supposed intellectual superiority.