I would suggest using Eastons Tuning Guide for setting Center Shot with the Plunger. Personally, when I use Easton Aluminum Carbon(X10), I set the plunger dead center shot. But if I use aluminum or wood, I set the centershot to the left (I am right handed), this is consistant with the Easton Guide.
For spring tension see the diagram below. Start by setting the plunger as tight as it will go and shoot at 10 yards. Back off 1/4 turn at a time and shoot some more (keep repeating this step, and record your spring tension). At a certain point the POI will start to change. This should continue for a few complete turns. It will stop when then the Spring is set too loosely. You are going to want to set the plunger at the mid point. See diagram below:
Hope this helps.