The one you have will work fine.  Not sure why everyone is so worried about closed center hydraulics the way its set up right now is how it should be used.  Its no different then an orifice going to hyd cylinder, if you have the tractor set at 4 or 10 its the same speed, thats exactly what you want to do.  The brand new spreaders have a control valve to set spinner speed, to set tractor side have spinners running at required speed, slow down tractor hydraulics till spinners start to slow down, then speed it up a little bit and go.