As I have been refining my script, I have also been developing a business writing workshop. My client and I are discussing this same issue: how do you convey the proper "tone?" Of course the first rule for business writing is the same first rule for all writing (whether it is a play or a speech): know your audience. Once you have the specifics of your audience in mind, you take some of the guesswork out of finding the right tone. But then you need to do what we do as playwrights, get outside your own head and listen to your message with different ears. And make no mistake: this is important for all types of writing, not just speech writing. You need to hear how your message sounds. Because when people read what you have written, they hear it in their heads. And if there is any possibility at all that your message can be misinterpreted, you need to rewrite it. Usually this means simplifying the sentence structure, and revising your word choice to use concrete language and active verbs. Sometimes it means tweaking your organization, so you clearly lead with topic sentences and choose your supporting points more judiciously. But you always need to "consider the audience" and how they will receive your message. If you write in a way they find oblique, opaque, or disrespectful for any reason, whether or not that was your intention, you will lose them.
So take a page from the playwright's script, and make sure your words clearly speak for themselves. Because you don't have the luxury of including stage directions!