To find the documentation on "offsets", click on the offset checkbox (just below the Delta X, Y, Z fields), then use [F1] on any input field.

Assume your 24" header runs from 110-120-130 with 120 the tee (cross) node. When you model the two branches, say 120-600 and 120-700 you want to use an axial offset of 12" (half the diameter of header).