Late to the game here, but I have some thoughts on trying to replicate the effect.

You do not need to memorize the deck. Since the second deck is cold, a simple stack would suffice. I do a similar effect, and use the Bart Harding stack.

Spectator one selects a number. I like 1-52, and simply suggest something greater than 1 and not "too big", and they shall see why later.
Using the BH stack math, you now now what card lies at that number in the cold deck.
Spread through the second deck face up, apparently to show the cards are mixed and cull the matching card to the top. Mix again if you like retaining the top card.
Force as desired.