Canister Head Pressure information:
Click below and for a good explanation Head Pressure and read the entire article here "Real World" Canister Filter Flow Rates:
Here is an actual test using the Filstar S (manufactures rated at 250 gph) and timing its flow to fill a container. Note: -34% just plugging the unit in level with the aquarium and NO media!
Level with the aquarium - 164.53 gph
24" below the aquarium - 153.00 gph (-7%)
52" below the aquarium - 142.87 gph (-13%)
These results were with a filter with NO resistance in the filter (all media was removed). The tubing was not cut, shortened or lengthened (as tubing length can also change results by adding more water resistance).
What is noteworthy is that despite this siphon aid, tests and 1000s of practical use applications show that there is still a drop in flow rate for a closed system canister filter due to friction, tubing size, devices such as UV Sterilizers, CO2 equipment, etc. along with some impact from head pressure.
Another example is the Fluval FX6 with a 0 head pressure rating 925 gph, when in reality the typical head pressure flow rate is 600 gph or less after the added resistance in the filter media and tubing are applied (as well as the addition of a UV Sterilizer).
Though it's far from an exact formula, a typical pump/canister filter flow rate with an under tank placement is about 50% to 60% of the published 0 head flow rate. This is the number you should use for mating your UV Sterilizer.