Its possible that with a very fine grind, like it sounds like you're doing, that you'll get conversion faster than a regular milled grain would. Part of the 60 min mash involves extraction of the starch from the grain fragments, but if its flour that will necessarily occur much faster. That being said, if you're trying to figure out the minimal amount of time for your mash with your grind, I wouldn't just use an iodine test as an indication that you can stop mashing. Iodine is great as a quick test, but a negative reaction doesn't mean that all the starch is converted, just a good deal of it. You also need to give the amylases enough time to properly process the starch fragments. While the starch may be broken down enough to test negative with iodine, its possible that its not been fully converted to maltose/maltotriose/etc. The only good way I can think to test that would be to see what your fermentability is at short mash times and see if it matches up with your mash temp, and to check to see if any haze develops from larger starch fragments.