Many people calculate them by using 500lph per 100g, some do 400lph per 100g. I use 4lph per g for low, 6 for med and 8 for high. Those with a nicely designed cylinder skimmer seem to be pretty accurate imo.
When you rate a cone its a bit different though. I generally use 3-4-5 So if you are using a 900lph cone vs 900lph cylinder you would have this. These are my opinions ofcours but they have been pretty right on in my experience.
Cylinder ratings of 225-150-112.5
Cone ratings of 300- 225-180