What is the formula to calculate slope for drainage pipes?
For example 1:200 slope for 200mm pipe?

In my experience in U.S. highway construction, drainage (storm) is normally expressed as a percentage.
I would interpret a 1:200 factor (although I have never seen it with drainage drawings) to be one foot of drop in 200 feet, or a 0.5% gradient, calculated by dividing the amount of fall (1 foot) by the horizontal distance, then multiplying by 100 for percent of gradient.