As you probably know, these experiments aren't of much use unless they're double-blinded, because of the incredible power of confirmation bias. As a step, you might write down very specific predictions, and also a bunch of specific things you don't predict as though they were predictions. You might do 30 predictions and switch 15 of them to the opposite prediction, maybe. Keep track of which are which, but use labels only you know (like a triangle and a square, say). Then ask others to help. Ask one person to rate each prediction on its specificity and surprisingness (for example, "you will meet a handsome stranger" is not specific, while "the sun will come up tomorrow" is specific, but does not require tarot to predict). Then have some other person look at all the predictions and fake-predictions you made, and mark which ones came true. Make sure they have no idea what the labels mean, and make sure you are not anywhere nearby giving subconscious cues while they do these tasks. You should just hand them the predictions, labeled in a totally generic way (so not plus versus minus, for example, which has connotations), and let them rate them. This still isn't a well-designed study, I would say, but you get the idea. Tarot's predictive power is only confirmed (some) if you get surprising, specific predictions right significantly more often than your non-predictions get marked as right.
Also: I don't have faith that if I let go of something it will (in typical circumstances) fall to the ground. I have very good truth-related reason to think this will happen, based on what I know about gravity, past such events, and so on.
Then spellcraft is reason, after awhile and repetitive work. The study sounds interesting for someone more concerned with the process and scientific analysis and less for something more interested in its practical applications. When you said leave before you give away subconscious cues, well, most of witchcraft is about harnessing and manipulating that subconscious. Not to say confirmation bias is the only thing going on in a tarot reading, many other things such as a woman unhappy with her relationship is eventually going to leave it just because that's the most probable outcome, basic predictions people who study psychology can do fairly easily, as well as confirmation bias. That's why the job of a tarot reader has to be taken seriously and with a sense of morality. We guide the people we read for using the philosophy of the stories in the cards, which is basically just this philosophy that things are cyclical and any end is a beginning, you know, basic uplifting stuff. Much of tarot reading is also reading the current state of the person, which we get confirmation of in the moment by their subconscious cue admitting what we said was true. And for specificity, we just look at how happy someone comes out of the reading, do they know something more about themselves or the path they choose to follow, or are they left thinking it's a hoax. If the latter, not specific or a good reading. Tarot isn't actually about predictions. It's one part current emotional state, one part meditation on a problem, and a very small spice of most likely outcome to come from the emotion and problem, which psychologists have been able to predict with these double-blind studies and authors because they just study and write about people all the time.