Part Setup:  Chapter 4 in the Centroid Manual covers this subject in general and in depth.  However, the plasma machines we are discussing here can be setup with the following simplified procedure.
On the 2100-series, there are three steps to consider, any time an operator wants to change material OD:
1.0.  Auto-Support Height setup (see Auto Support section)
2.0.  Front Material Support Height (see Material Support section)
3.0.  Z-Height/Torch-Height (see below)

2100-Series Machine X-Axis Set-Up (done at the factory, this should only need to be redone if someone mistakenly resets it.  It is NOT a normal part of changing material diameters):
1.0 Remove any material from the chuck.  Jog the X-Axis all the way to the torch end of the machine.
2.0 Be sure WCS#1 (G54) is displayed in the upper left-hand corner of the screen.  If not use F6 or F7 in the Part Setup menu to switch it to WCS#1 (G54)
3.0 Jog the torch (Z-Axis) down near CR-A.
4.0 With a tape measure, measure from the face of the chuck (not a jaw, not in the center bore) to the center of the nozzle on the torch.  Take the measurement twice and be sure it is accurate to better than 1/16".  Convert it to its decimal equivalent and write it down.  (On machines shipped since late October 2018 this value should be 10.00in.)  This value is critical to a feature called ROP (Reverse Order Processing).  Please read that section as well.
5.0 Now from the base level menu press the following sequence  
5.1 F1-Setup  
5.2 F1-Part 
5.3 Note that by default the X-axis is selected.
6.0 Enter the value from Step 4 in the first field
7.0 Enter 1.0 in the second field
8.0 Using the space bar to toggle the value in the 3rd field, Approach From, to "Center" if it is not already.
9.0 Press F10-Set
10.0 Note that the X-axis DRO now displays the number you measured.  If not try again.
11.0 Now hit ESC a couple of times back to the base level menu.
12.0 Open PieceMaker and go to the Parameters Page.
13.0 On the center tab, Machine Parameters, the 3rd value down is Stub Length.
14.0 Enter the value from step 4 there and hit SAVE.