For proper chain slack measurement, do you measure the 12mm gap to the bottom of the chain slider or to the bottom of the swingarm. I'm guessing the 12mm gap is from top of chain to bottom of alum. swingarm. If I'm wrong, it will be too loose.
I adjust mine so I can just slip three fingers between the chain and swing arm at a point adjacent to the inside shoulder of the rim, with the bike on it's side stand. Works for me.