Here's a quick thought.
How about using cards?

Have, let's say the 4H eighteenth from the top of a deck.
Make a prediction.
Have 3 number cards chosen below the 18th from a spread or maybe false shuffle and cut the deck in half then use the Magician's Choice so they get the bottom half, no pairs allowed.
Place the former top half on the bottom half or leave it on the table.
Do the necessary calculations to where they get 1089.
Then have them eliminate the zeros and add 1+8+9=18.

Like I said, a quick idea.

I'd be interested in other forces of the 3 digits using everyday items. Have any thoughts?

Without going into too many details, but you could have a double prediction prepared in an envelope with 198 or 1089, another option is mental reading, when they do the first calculation they find out if they have 2 or 3 digits, then they add and finally the reading is done. .

Hello Jaz,

Having the participant work all the way through to 1089 is more work than is really needed. The participant can stop right after the subtraction, then form the digital root. It's always guaranteed to be 9, so that's where the force card would be.

Note 1: The notion of a digital root is common in numerology, so your script could make use of that fact to justify it.

Note 2: This works with any number of cards greater than one, not just three. That would give the participant even more freedom.

Note 3: The mechanics of this force (as opposed to the 1089 force the original poster asked about) date back at least to Paul von Kalnassy, "Mnemotechnik: Gedanken-Erraten," Der Zauberspiegel, Volume 4, Number 6, June 1904, pp. 98, 99. A half-century later it was popularized by Martin Gardner in his Mathematics, Magic and Mystery, (New York: Dover Publications, Inc., 1956), p. 165.

Anyway, this might be a quicker way to get the arithmetic over and on to the magic.

Thomas Henry