The predictions will, of course, have to be true, even if the characters woefully misinterpret them. Great rule in fiction: no matter how relentlessly mundane your story is, when you tell your fortune, it has to be true. Few plot twists more difficult to pull off than a falsified prediction, even if few are harder to work with than a true one.
Vague. Vague is good. Especially since the techniques used are good at vague, which lets people put their wishful thinking through its paces. "Prosperity" -- "fell magic on the loose" -- "battle" -- so much room for imagination.
But on the other hand, I also devised more than one technique. To characterize. Some characters use the old ways, involving stones with special virtues, and others traffic with spirits. Except that when I started to write one of the trafficking ones, it involves the stones. . . perhaps I ought to revise it even in the outline to make them more distinct.
