The default setpoint corresponds to a percentage of full scale. You should look at the manual for the PID Toolset. Specifically, on page 3-14 there is a section called "Converting Between Percentage of Full Scale and Engineering Units."
As far as manually accessing the data in the array of clusters, I'd wire the array into a For loop so that it changes one element with each loop. Then I would use a bundle or a bundle by name to change the data in each element. Wire that to the outside of the for loop to create the new array. I attached a simple example