California 
Thank you all for your help!
I think I’ll be going with “Business Formal” as it seems to be the most accurate of them all: Jeans are a definite no–no, khakis don’t cut it but a tux is too much.
Ook.
You do realize, of course, that that cite is from a guide published nearly a century ago, right?
Miss Manners says, in 1999, that "full evening dress (the evening tailcoat), now called "white tie,“was, before that, merely formal dress.” “When “black tie” was invented,” she continues, “it was demi-toilette or informal evening wear; now black tie is considered formal wear unless white tie is specified.”
And, very importantly for those gentlemen who do not wear formal wear that often, “[T]he dinner jacket [i.e., tuxedo or black tie jacket] was invented before there was a shirt to go with it… Now… it does have its own shirt, a pleated shirt with a fold-down collar… Miss Manners will stop short of saying it is wrong [to wear black ties with collars that do not cover the neck], and confine herself to saying that gentlemen with their black ties exposed all around their necks look silly.”
And the invite in the OP should request “business attire,” which means a suit and tie.