I think if you already have shirts and pants in shades of brown, you’re good to go. That’s a beige-gold that will go with all kinds of browns. I’d do dark brown pants and lighter shirt (that’s darker than the tie). And probably wear the tie loose with the top button undone, since a tie is probably a little dressier than the event with no jacket required. (That could be me, though, I think tied up ties look weird without a jacket.)
I’ve been a bit busy, but to answer your question, no, there isn’t an accepted definition of “shade”.
I am not much of one for “It’s more of an art than a science”, but that is the truth for colors. The last time I had to pick a paint to match an existing color (usually I’ll use one that gives a pleasing contrast), it took me three months. It takes me hours to pick a shade of white paint. White.
And language doesn’t help; is ivory or cream darker?