Claptrap: thanks.
Sebastian, Lorax: you're certainly right, there's no formula. (This is not, thank god, the writing section of the GRE.)
What I'm after is, more, whether there's anything slipperier than a formula -- a tone or type of decorum or sense of audience or... something -- that a lot of successful SOPs have in common. Whether there's anything about those SOPs that it would be useful to get a feel for. Probably, mostly, I'm being neurotic and looking to reassure myself that what I'm writing is in the ballpark of what's been acceptable in past cycles. This process seems to make everyone neurotic, so maybe other people are, too?