OK, is 254mm to the bottom of the light, or to the pipe itself?
The length of the curved part would be one quarter the perimeter of a circle with the same radius.
254 x 2 = 508mm diameter circle. 508 x 3.14 = 1595mm / 4 = 398.78 mm for the curved part.
From there you can just add on the straight section to the floor and any straight part at the top that you may want for mounting the light. From your drawing above it looks like 1350mm for the straight shot to the floor + the 399mm needed for the curve + say maybe 100mm straight section at the top to mount your light too. In this example you would need a pole 1849mm long. (Just over 6')
Assuming you want the bottom of the light to be 10" of the water, then the radius of the curve obviously needs to be more than 254mm. Add on the thickness of the light and any mounting hardware. So if your light and hardware is 50mm, go with around 304mm radius in the equation above.
Also note that this math follows the center of the pipe. So if your pipe is 1" thick, you may want to add 1/2" to the "thickness" of your light and hardware as mentioned above, to account for the half of the pipe that hangs below the center-line. That would bring the radius of your curve closer to 316mm for a 2" thick light that had it's emitting surface 10" off the water.