Hi,
I suggest you search the wobbly wide web.
The rule works something like this. (I used 120 days to make the math easier).
120 day stay in 2016, 1/3 of days count towards 2017 stay (40), 1/6 of days count towards 2018 (20).
120 day stay in 2017, 1/3 of days count towards 2018 stay (40), and 1/6 towards 2019 (20).
If you stay 120 days every year--then you have maxed out your 180 day welcome. i.e. 120 +40 +20 = 180 days.
I full time, and do not have a sticks and bricks in Canada. That means I can't vote. Therefore it behooves me to follow the rules carefully.
I do own property south of the border which again means I have to be careful.
Can you get away with more days? The answer is a resounding "maybe", if you don't get caught.
Remember with the new passports that came online in June of 2013, they know EXACTLY how many days you spent in the south.