That's a very small hole (2mm??)- try a liquid orifice plate calc - there's plenty on the web but you also need to kknow your fluid density and what pressure you're running at. Initial numbers are around 1 GPM so your pump will soon stop working or you're throwing money away throttling and re-cycling and then the fluid will heat up. Are you sure these numbers are right??
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
Take the max pressure of the pump flowing through that size orifice, and the flow you get should be about 99% accurate (for the short time period it runs, until the pump is damaged.)
I do not have my diagram but it is pretty straight forward : (im on my iphone right now)
It is the hydraulic system of rear-end garbage truck. The 1" hose goes to a valve that has 1 inches outputs. We wanted to limit the speed of an hydraulic accessory, so we put a restriction on the output of this valve. It does have a recirculating loop by the way.
It works perfectly, we managed to limit the speed of the accessory with the reducer, but now the client wants us to add velocity fuses. That is why i need to calculate the GPM/speed through the reducer to find the proper velocity fuse.