I want to use a plough plane (stanley #45) to cut dadoes (cross grain). The spurs/knickers are useful in preventing tearout and defining a clean knife wall, but they score past the bottom of the dado, like in the following image (dado profile outlined in red).
This is obviously due to the knickers cutting ahead of the iron, but if push the iron further out to a depth equal (or close to) the spurs, the operation of the blade is much less smoother -- broken shavings, and at the far edge, it results in breakout.
So, is there a way to produce perfectly rectangular dado profiles with this tool? Would one re-adjust the depth of the blade relative to the spur, or are spur marks to be expected regardless?