I added a tablespoon of wheat flour (during the boil) to my last wit, and the stupid thing has dropped crystal clear after a couple days in the fridge. How does one go about getting starch haze? How much starch does it take?
No idea! I've never tried to add starch haze.
It isn't very soluble, though, so it will drop out of suspension. If the OP is bottling, however, it will probably be more apt to get stirred back up and into the final beer. It's possible that the smaller starches and larger, less-soluble dextrins resulting from a mash might stay in suspension longer than those produced by boiling undigested flour.
If you want intentional haze, another approach might be to use something like flaked oats in the boil (which seems appropriate to a Wit), or the mash, of course. Beta-glucans are a widely ignored source of potential haze and they have a more diverse solubility than starches, which might keep them around longer.
One thing the OP can do is pour a cold beer and let it warm up. If the haze doesn't go away, then the first place to look is probably starch or yeast haze. If it does, it is probably proteins and polyphenols. The most direct prevention for the various types of hazes will be different, but cold storage for a week or two will work for most.