Here is a bit of info that you may or may not have:
www.nibco.com/showFile.do?fileId=2626
The valve is used for adjusting the output temperature to various parts of a heating system, which may or may not have any use in a system such as your's which appears to be solely a radiant slab.
It's primary application, according to the link above would be in a dual purpose system such as a boiler system providing heat to both baseboard heaters and radiant floor, or baseboard and domestic hot water.
Baseboard systems typically call for 180*+ water while radiant floor systems only require 110-120* water. When mixing the two systems the valve mixes return water from the floor system with the supply water to limit the temp of the water going to the floor.
SO, as you may have guessed, in a radiant floor only system such as your's you likely have no use for the valve, maybe.
Most radiant floor systems call for approx. 110-120* water into the floor, and expect to the return water to be approx. 10-15* cooler than the supply. The return temp is controlled by the speed that the water is moved. Too much temp drop means the water is moving too slowly, not enough means the water is running too fast to release it's heat to the floor.
I would start with the mixing valve fully closed, max temp output, and go from there depending on how your system performs.