You have ml, shouldn't that be mm?.

A fun way to do this is with calculus. You can find the area of the water to a depth of .4 meters by using:

Can you integrate that?.

To find the wetted perimeter, you can use the formula for the middle ordinate.

The middle ordinate is the distance from the top of the water to the top of the pipe. 100 mm.

Solve for theta and get

But we want the wetted perimeter. The above is the perimeter of the dry portion.

Now, we can plug that into