I've only used wheat flakes in wit, but for a Kolsch I think 8% would be enough - certainly not too little. I assume you'll be toasting them yourself? With oats you want to toast them and let them air out for a few days before use to let some volitile compounds mellow out. I assume the same is true for wheat. At a minimum, don't do what I did the first time I toasted oats. Threw them straight from the oven to the mash. 1lb of 350F oats will really mess up your mash temperature!