Disclaimer: I’m a bit tired, so forgive me if I misunderstood what you’re trying to do 
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[Drag Selecting multiple nodes)
The post linked above is actually meant for drag selecting, but it’s major component is what I think you’re asking for – finding out where on the screen’s surface a 3D object’s center is, if I can put it this descriptive way. If I understood you correctly, it should provide a nice foundation for you to work on.