You got the math mixed up. How did you get 200cm (actual) = 1,000cm (minifig scale)?
If 180cm = 3.5cm, the ratio is- 180cm / 3.5cm = 51.4
So to get your cannon's minifig range is- 200cm / ? = 51.4
? = 10,280cm is 102.8m giving your cannon a little over 100 meters of firing range in minifig scale.