The miner falls when the slope is steeper than the 2:1 slope she would produce. There is minimal leeway: You can mine over a 2:1 builder's bridge that is rougher than the hi-res 2:1 miner slope, but overall it's not much.
I agree how the early miner cancelling is strange. I could allow extra leeway on the first swing, but that's special-casing, I'd like to avoid that. Hmm.
-- Simon