How to taper an irregular flat roof for a slight fall
The slope of the roof is 2/100 so the elevation of the roof at and given distance from the drain can be expressed proportionally 2/100=elevation/distance or e=(2d)/100. So yes, 236 from the drain the elevation is (2*236)/100 = 4.72. If you can measure with a tape or step off the distance with dividers you can dispense with any more math than that. 

Basically on the building you can measure the distance from the drain to any point on the roof and mark the height. Or if you're trying to cut strips you can find the start and end heights as above then snap a chalk line between them.

As a practical matter you'd probably want to increase the slope slightly beyond code and pick a few critical points such as corners and the midpoint of long edges to set the height. The reason being that you probably a) can't build a perfect cone with construction lumber and b) the rest of the building probably isn't perfect either. Giving yourself a margin for error means that every point in between should have at least the minimum slope.

By the way, the angle is irrelevant to the Pythagorean theorem which states the length of the hypotenuse is equal to the square root of the sum of the squares of the legs.