This is one of the Fluid Power hydraulic formulas you can use to calculate how much heat you need to remove from the system. If you don't know how much horsepower you are using in your system, you can find the formula for hydraulic horsepower here, then you can remove a certain amount of that horse power with the hydraulic calculation below in this video.
Hydraulic needle valves can be a low, medium, or high pressure needle valves, they all work under the same principles of restricting flow to the desire amount depending on your requirements. Many manufacturers may build their own system on how a needle valve might work internally. Here is one example on how a needle valve would work.